Spike sometimes stays in his cave for days at a time the only way to get him out is with food and sometimes he just eats and goes right back in. If he spends the night in it he won't get up in the morning
Brumation most likely. . . give hime a bath and make sure he poops, if it all looks normal, don't "force" feed him and just leave hime be. As long as he doesn't look like he is losing weight and eyes are clear I wouldn't worry. The first time my went into it I thought I was in for a dead dragon. I just kept an eye on him and now he's full of energy!
Read the articles on this forum about it and see if it sounds like what's going on.
I was going to post the exact same thing! Smaug is doing the exact same thing. Luckily I managed to read up on brumation from the sticky, and it sounds like our dragons are both going down for naps I really suggest you read it too, it really helped me out.