Sony Vegas help, please! :(

I'm making a small video for a song and I've spent all evening doing it, no where near finished. But I added something, or done something and suddenly half of my screen is green. I cant find anything that's the cause of it. I done 'undo X', lots of times - even carried on doing it before the green part appeared but it seems to have done nothing. And me, being an idiot - clicked Save rather than Save As, so I dont have a backup or anything.

I'd show the video, but I don't know where to upload it etc. If anyone can help me, even if it's getting back my copy before I done the latest save. I dont want to loose all the stuff :(

Try Ctrl + z

That's what I was doing, to undo it. Like I said, I tried to undo even past the stage from where the green box appeared but it still wouldn't get rid of it T_T

Have you got anything above your video on the timeline? if not it could be the Chroma Key, which you can edit by going here:
Video FX >> Chroma Keyer