could be an electrical shortage. check your circuit breakers. there is also the possibility of the bulb in your t.v. burning out. maybe time to replace the bulb. The best possible advice would be to find a business that specializes in zenith projection tv troubles.
Most likely there is some sort of loose cable. Check the power cord first, then any other cable connected to the TV like cable box, DVD player, VCR. If everything checks out then it most likely needs to be repaired.