I've had this problem frequently over the years.
I'll go to put text into a picture, having selected the text color, I start typing, and the cursor moves as I type, apparently recording my keystrokes, but it's invisible. Selecting a different color for the text doesn't make the text appear. The only "fix" I've found is to close Paint.net, start it up again, bring in the file and use the text tool again.
It seems like a silly bug of some sort, but maybe it's user error or some sort?