IPod Classic - music gone
My old classic used to do that kind of trick to me periodically.
I always suspected that the disk format was not particularly robust.

I found the only way to get it going again was to Restore it in iTunes &amp; reload the whole library again - don't plan on being in a hurry, it takes ages.

One thing I did learn over the years was to always leave the lock button on [the little slider at the top, slid to show the red dot underneath] all the time unless I actually needed to interact with it.  

Also to never leave it locked if I was about to connect it to iTunes, otherwise it would hang.

Remembering those two would let me run far longer before it played tricks on me the next time.