I opened a drawing I had already made and saved as an .svg file. I want to change the fill color but in Fill the color is listed as undefined.

This is a simple rectangle with black text and some blue fill with a gradient I want to change to red.

Is the color locked in as blue now and unchangable?

I'm pretty sure it shows the fill as Undefined, because you're using a gradient for the fill. You can change it however you want, as many times as you want!

It sounds like you've selected a group or clip or something. What does the status bar say you have selected?

Groups and clips don't have colour properties, only their contents have colour.