[System] How do you troubleshoot a freeze-up?

[Shibblet]

This has happened quite a few times on me now. Just a freeze-up in the middle of doing things... Mouse stopped moving, and I had no choice but to hard-boot the computer.
It's usually not a big deal, I just deal with it and move on... but now that it seems to be happening more frequently... and moreso, that I really don't even know how to begin to troubleshoot a "freeze-up."

[Qqmike]

Big, good question.
I'll drop kind of a standard list that comes to mind, until someone pipes in.

I've never tried this for a safe reboot:
Alt + SysRq (orPrint Screen) keys at same time, then one by one, slowly R -> E -> I -> S -> U -> B.

htop tells you your resource usage.

system logs: journalctl -b -1 -k

People often suspect system drivers, especially graphics drivers.

Hardware:

Is your PC overheating?
Clean out your PC case if there's a lot of dust.
Is it your power supply?
Is it a memory stick? Maybe use Memtest? (I've had this problem before -- a cracked module)
Is it a cracked motherboard? Jeez, I hope not, but I've seen that.