Scratch

I have been working on a project for a while and it has been going fine. But after the last I saved it, I turned off my computer and when I turned it on again I tried to open my project again. I opened scratch, pressed "open" and selected my project. But then a message popped up that stopped it from opening

Could not load:
Key not found

I am wondering why this keeps happening and if I can fix it. (Luckily I do have a back-up file that works, but it is not quite so updated. I can use it if I can't fix the problem but it would be easier if I didn't need to.)

Can anyone tell me why this is happening?