The aluminum head may have warped, causing the head gasket to blow and the valves to get damaged. Compression is then shot. Typically, this is what happens.

If you think this may have happened, remove all of the spark plugs and record each cylinder's compression.

Well besides a blown head gasket, what else might cause this? A bad thermostat? Waterpump? How do I check those

Perform the tests in the link below:

https://www.askmehelpdesk.com/cars-t...tml#post229012