Hmmm... A POST5 error caused by a bad SD card usually does not result in a black UI screen.
Is the UI BLACK or have a blank GREEN screen?
POST5 is usually resolved by replacing the SBC, but sometimes the SD card as well (at same time).
Then of course Altbooting the software. Make sure you use a sw version later than mid 2018, otherwise the POST5 can occur again. The issue causing the POST5 was resolved by sw after about spring 2018 or so.
All the buttons that turn green are stay on when the machine is turned on but the screen is blank. What could I try to get it working.