Hard drive renamed after restart, apps can't see it
I would run basic permissions &amp; disk checks, just to be sure, but it wouldn't be the first time I'd ever heard of someone [or their cat] accidentally renaming a drive by leaning on Return &amp; a few keys, without being aware that the HD was selected at the time.

If you find no disk corruption I'd work on that assumption for now - but I'd also make doubly sure of my backup strategy, on the chance it's a sign of an imminent failure.