I've had this happen twice and my HDTV is a Sony. This is a brand new, ATT updated by the tech Uverse box.
Looks like Uverse has some bugs.
Using a HDMI cable to connet TV to ATT Uverse set top box ( set top box mfg is Motorola).
Periodically, when I turn both items on, no picture but audio comes thru. Then must go thru ridiculous process of unplugging all cables, powering TV and DVR/Settop box off.
Also, when this is happening, pressing the menu button on remote also results in no picture...not even getting 'white snow'.