I had a similar problem recently when I upgraded computers from a Macbook Pro to a Mac Pro and then tried to open older projects from an external hard drive on the new system. The project that I was trying to open was a short 3 minute HD clip, but there was tons of HDV footage associated with it, layered on top of each other with lots of animations and renderings already complete. I had worked on this thing for almost half a year, teaching myself the program from A-Z, then set the project aside to work on something else only to come back to it tonight for the first time in months. But the project wouldn’t fully load. It kept getting stuck at 90% and then everything would just stop. The Activity Monitor showed no activity. I let it sit for 30 minutes. Still nothing.
After freaking out and trying various suggestions that didn’t work, I came back to this forum and saw the Vincent’s suggestion about moving the CAPTURED media folder to a new location on the external hard drive, forcing Premiere to ask where the files were. It’s a smart trick. And it worked! With a little tweaking.
Through this process I found that some of my rendered files were lost. I have no idea what happened to them, but as long as you are able to direct Premiere to the original media, you should be fine. You will have to re-render the entire project once it opens, but the work is not lost.
Now, here’s the trick I discovered: Once you finish pointing Premiere to all the media and available rendered mpegs, the project will load. But it may immediately crash the program, forcing you to start over. The trick is to USE YOUR SHORTCUT KEYS (COMMAND +S) TO SAVE THE PROJECT BEFORE THE PROGRAM CRASHES. The program may crash while saving, but as long as it starts to save, you should be fine.
Restart the program. Open the project. And there ya go! You should see the project as it was, only unrendered in the places where the render files were lost. Set it to render overnight and you’ll be back to where you left off next time you open the project.
Hope this helps!