There is a high likelihood that something happened to your poor guy. Maybe an unknown punishment from a family member? Or, as Den said, maybe something changed in the house. I would DEFINITELY take him to the vet to make sure nothing is physically wrong, such as cataracts or otherwise. It is a HUGE jump for him to go from happy-go-lucky to petrified. If the vet can offer no answers, call a behaviorist in your area immediately.