iMovie won't play projects/work at all?

I tried uploading a project to YouTube one day and it uploaded as a black screen. I went into iMovie and it wouln't open or play any of my projects. I tried starting a new project, but the screen shows up as a blank grey screen when I play it. I tried deleting the .plist file in Finder and it doesn't help. I've also tried seeing if anyone else on the internet has the same problem but deleting the .plist file seemed to work for them. Does anyone have any ideas as to what I should do or what went wrong?