A lot of Sony Bravia have a faulty firmware that shuts then down after using the television for 1200 hours. Really!
The television can be temporarily reset by disconnecting the AC Power Cord from the wall outlet, wait at least 30 seconds and then reconnect the AC power cord to the wall outlet. After this reset procedure, the television should work properly until another several months of use has accumulated.
For a correct firmware upgrade contact Sony directly.