My friend has heat pump issues and asks me for help, here are my homework and hopefully, I can get it fixed and update my note afterwards.

This guy gives five reasons that the thermostat can go black, I find it very useful and summarize the issues below.

1. Protection system kicks in the "shut off" the system, i.e. the drain pan is full of water
2. High voltage power issues, i.e. after power supply outage
3. Low voltage power supply issues, transformer's failure etc.
4. Battery fails
5. The thermostat itself fails

This guy makes the video thermostat wiring with heat pump system.

Good sources of videos

EdgeTek does a good job providing useful information.

Check the freon level

A common tutorial.
This one presents measuring in heat mode

The video gives clear explanation how the reverse valve can fail, and some important diagnosis strategies.

His initial video on basic heat pump operation, this is very helpful.

Diagnose the reverse valve.