I assume you are using the correct HDMI video output on the 800 and not the audio only one...Otherwise, after the player is on, have you tried holding the power button for 10 seconds to turn off, release and then turn back on? Or maybe holding it for 10 seconds on power up ...just to see if that does anything...So far been happy with my sony 800. Recently started to lose handshaking/blank scree but resolved by unplugging and plugging HDMI cable.
Cable used Certified full 18gps 4K Philips cables.
Now no picture. No signal. Plugged direct to LG tele with other cables all similar 4k and no picture. Do get sound.
Tried on another 4k TV and nothing. Only get a brief "Sony" on start up. Draw opens closes fine.
Can't reset as can't get to Home screen. There doesn't appear to be a way of hardware reset.
Player out of warranty but hardly used.
Any help as doesn't seem to be a lot of info on what appears to be a common issue.