Here's a strange question.
I tried to change the resolution of my game. (Probably a stupid thing to do).
I changed it to 640 x 400, after already having worked on it. Then, without doing anything, I changed it back.
Now, it refuses to let me test the game. Is there a way to fix this? If so, what?
Try the 'recompile all room files' option from the menu. That might work.
Otherwise, restore from a backup?