Hello, I'm new to Inkscape and I have a question. I copied and pasted an image from the internet and then used the "trace bitmap" function and it worked. I have tried to do the same thing several times since and nothing happens now. Is there a setting or something I accidentally changed?

Just an outside possibility - sometimes the Trace is so good that you think nothing has happened because the vector trace is sitting on top of the bitmap and looking exactly like it. Deselect everything by clicking away or hitting esc nd then click on it again and see what the status bar at the bottom says you might be surprised to see that it says 'path with 3000 nodes' or 'group of 5 objects' instead of 'image 300 x 400px'!

Also make sure to actually select the object before trying to trace, it's easy to deselect and not notice. Happens frequently.
If that's not the issue, then I think it could be a bug - what's your Inkscape version? Operating system?