Corrupted .PRJ?
MikeM's response re shutting your system down before giving it enough of a
chance to finish writing the cache is just one of the many ways to get a
screwed-up file. None of them have anything directly to do with 3DS… they're
all system-related.
Note that (also as suggested) if you've got the backup file system ON, there's
a chance that your PRJ.BAK file contains the data you need (but that would only
be the case until you've written another PRJ file).
– G