Mike seems to be experiencing clinical depression, and he needs to find help right away. Mike has no energy, no interest in the things he used to like, and has lost his appetite. He's also obsessed with thoughts of hopelessness, has difficulty thinking straight, and wants to be alone all the time. All of these things are signs of clinical depression.
Mike may not even know how this depression started. It may have started with a tragedy that he just couldn't get over, or it may just be due to the natural brain and body chemicals that control human emotions. The good news is that Mike can get treatment in the form of therapy, medicine, or both, and can start to work his way out of the depression.