Possibly, it could be HDD failure. How old is it?
With the iPod classic (and I'm writing as a big fan of same) one of the problems was that a small, but consistently steady percentage of the classics suffered from complete HDD failure.
Actually, two of my iPod classics died completely on me - each time the HDD just died; fortunately, at the time, they were each still under warranty, and Apple replaced them without a murmur. Indeed, this excellent customer service was one of the key reasons I switched to Apple.
However, much though I love the classic (and I'm one of those who likes to have my entire music collection to hand in one portable device), I have to concede that a SSD (rather than the older style HDD, which is what it is equipped with) would be better, and more stable for the classic.