Sony XBR 55X850C won't start

Pretty much all modern TVs use switched mode power supplies. The capacitors in them eventually wear out, then the supply won’t “startup”. You mention a firmware update, if it died right after, that’s one thing. If it ran fine for a number of days after, the update probably isn’t cause. On pressing the power button, one flash of the On light points to supply startup failure. This is usually accompanied by a soft audible “tick”. Multiple flashes is a diagnostic code. Count the flashes, look up the service manual and see what it indicates. Supply startup repair is a matter of swapping out specific capacitors, typically any and all 10uF+ right near the transformer. If you know how to work with circuit boards and solder, get new capacitors (match uF, temperature and voltage greater or equal) and have at. Otherwise document what it’s doing and call some repair shops for estimates. I’ve fixed several TV and computer monitors by swapping capacitors, it’s about $10 parts and ~1 hour labor..