How new are you to Linux?* /gen
Have you checked the output of, say, “journalctl -b -2”? This would give you a system log of messages that go back to two boot sessions ago and normally show you what happened just before that first shutdown.
This could be drivers, background processes, hardware, no way to know without starting by looking at logs. Hell, I’ve had a monitor crash out my system multiple times and I wouldn’t have ever guessed if I didn’t check my logs and dmesg output.
* to get a better sense of how to help you