What's the most likely cause of this? I didn't move anything and other, much smaller projects I open containing the same files are fine. Right now, I suspect that I'm importing so much into this project that it's slowing the project down. It's been freezing on me lately.
I hope it's a temporary glitch and the files will come back soon. Otherwise I have to literally pull each and every file from my drives and replace them all :/
The most likely reason is changed drive letters. Make them fixed in Disk Manager.
Tell it to reconnect them all - and it should ask you where they are.....