Not a happy day! An Inkscape .svg I have exported before now will not export. I've tried roughly every 1/2 hour for 5 hours to export. Neither will it SAVE AS into a new name - it turns the new file to scrap. I can copy the file from the outside and paste it and rename it but the renamed copy won't export either - it will let me make and save changes.
What might prevent a file from suddenly not responding when asked to export?

My guess would be your file is extremly detailed.
Or it contains a problematic part.
Have you tried to divide it into two halfs?
You mentioned copy-paste. If you copy and paste one half in one document, the other half in another, and try saving each, maybe it would work.
If not, repeat this until you will find which part of the svg makes the error.

I hate to state an obvious, but I will, be sure that you've closed down everything else so that all memory can go into the export. Then, close down Inkscape and reboot, you can try just re-starting Inkscape, but if that doesn't do it, re-boot your entire computer. I know that I've spent an hour trying to export something, got strange errors, then after someone told me to re-boot, it worked perfectly.... (I know that when my CPU usage gets to even 60%, Inkscape files will crash and not export correctly).

You could attach the SVG (or otherwise make it available for inspection). If it's something wrong with the file, maybe one (or more) of our wise Inkscapers can spot the problem?

Thanks everyone. Yes, I tried shutting everything but Inkscape, and have closed down the comp several times, no luck tho. I haven't attached an .svg before. Do I attach as if it were any other file, or are there special requirements?

Only the file size is limited for svg-s.

maximum allowed size is 256 KiB.

If that's bigger, you can use dropbox.
Upload the svg to your dropbox, then get the share link and post it here.

You can attach it just like a regular file. If you get an error that it's too large, you can either zip it and attach the ZIP file, or upload it somewhere else.

Size is 27.2 MB.

Well then the reason it's taking so long to export, is because it's so huge. No need to upload. How much RAM in your system? Less than 4 gb? Problem solved. Right?

Try this. Some night, just before you go to bed, turn on your computer, and disable any automatic shutoff that you might have configured (assuming this isn't a laptop running on battery). Open Inkscape and open Export Bitmap. Make sure it's set for 90.0 DPI. Click Export and go to bed, leaving the computer running. Maybe you'll have a new PNG in the morning.

(90.0 DPI is what you should use, if you want the PNG in the exact same size (dimensions) it is now. If it's set for anything higher than that, it's only going to make the problem worse. If you need a larger export, you'll probably have to find a computer with more RAM. Or if it's a one-time thing, maybe someone here who has a massive system wouldn't mind doing it.)

If you still think there's something wrong with the file, you're welcome to provide the file.

Do you really need your file that big?
If you have an auto-traced image, you could divide the node nuber by up to ten times when simplifying the paths, usually with not much effect on the actual look.
Or is it a set of many small and detailed objects, like icons?
In such case exporting each object into separate files would be a better idea.
Or is the drawing that you really want to have one by one?
If so, you can still try to divide the exporting area to sections and stich the images in a raster program.

Generally there are some topics on adjusting inkscape a bit to your computer.
And you could try the 0.49 version which have a better renderer.

If you try a one-nighter export, disable any automatic downloads -program refreshing-, and disable screensaver as well.

Thanks everyone. I'm still working on it! It's a hand-drawn file, a mosquito with thousands of little body hairs, pond, moon reflected into pond, plants, rocks, sand to the horizon, stars in sky. It did export before with no probs. I've exported it heaps of times then come back again and again over the months making small adjustments - every export always successful. Til now.
I've begun trying to export minus one grouped item at a time - ie: Remove pond then try to export. Remove moon reflected onto pond then try to export. If something is dodgy it should reveal itself. Then I can replace that grouped item from its outside point of creation. (The file I'm trying to export is a bunch of stuff all purpose built in other files.)

Next day...

Interesting - the moment I now export this particular file it's turned to scrap and the export fails. I've ended up taking a screen grab as recommended in a post above and I'll use that instead.

PS this one above my level of skill "you can still try to divide the exporting area to sections and stich the images in a raster program."

What do you mean by 'turning to scrap', are you getting an error msg, etc? Also, if not getting an error msg, be sure you check which directory your export to file is. Sometimes I think that I've selected a particular folder to export to, then when I go to check, realize it's not thing, and think it's an export issue, but really, it's a 'dee' issue, lol.
Sorry, the 'divide the exporting area to sections and stitch' is a bit above my skill level too.
Have you tried opening the svg file in another program directly, like GIMP, then while in GIMP export it as a png?
dee

Resolved! Solution came about in the same way as my patchy export post problem, my reply to that one being "Success! I now have the Inkscape image exporting as a .png with all info appearing in the export. I achieved this by altering the file size from roughly original size @ 3962 x 2911 to 1357 x 1042, and then exporting.

The original size came about during the process of teaching myself Inkscape and getting very stuck, then using a couple of youtube tutorials to begin filling in the gaps. In one of the first tuts I watched size seemed to have no relevance, in fact it was suggested turning off the A4 box all together from the new blank canvass. So I did exactly that. And I proceeded to begin drawing in a random-fashion in terms of size. I based every other drawing on the same premise. Now I know better. Initial size is critical. So that's why all my figures in posts have been so extremely large. I had no idea at the time what those sizes meant until reading reply posts.

So thank you everyone who replied - it's been of massive help to me, I'd still be lost in the dark without all your replies and kind help."