Final Cut Pro X "The document “___” could not be opened" problem

Hi, I've used Final Cut Pro X for a long time now and I have never had any problems. Now, when I'm trying to open a project file it says "The document “project name” could not be opened." I worked this project two nights ago and it was perfectly fine. I tried restarting my computer, updating my computer and opening other projects. Some projects open some have the same error. How can i fix this? This project file is very important.