If it lasts longer than a couple of weeks and you lose interest in everything, find yourself wanting to stay in bed rather than go out, and feel generally hopeless it's depression. I've been there. Don't wait to get help.
Cabin fever I would say is somewhat seasonal and happens when weather or other circumstances prevent you from getting out and doing the things you enjoy. Goes away when you're able to resume your regular activities.
Mid life crisis - not sure how I'd define that one.
The important thing is if you feel like you need help talk to someone. Don't feel like you have to "get over it" all by yourself.