This has happened a few times, I would be working on a project, InDesign crashes, but doesn't recover it. Instead when I try to open the file, it says it can't be opened because it's already open by another user (there are no other users) I'll restart my computer and whatever to see if it will fix itself, but it doesn't. Does anyone else have this issue? Is there a fix?
With InDesign closed, look in the same folder where the file is stored and find its "lock file." It will have the extension .idlk. Delete it. Launch InDesign and retry to open your file.