If things are so frozen that you can't open System Monitor (task manager), try this: Hold down SysRq and Alt, let go of SysRq, keep holding down Alt, then press these buttons in order (don't need to hold them): REISUB - to remember it: "Raising Elephants Is So Utterly Boring", also known as "raising the elephant".
On some systems you need to enable this feature first, so try it out once to see if it works and look up how to enable it, if it doesn't.
https://en.m.wikipedia.org/wiki/Magic_SysRq_key
On most distros, you can also get into a command line, with the combination CTRL + ALT + F1 - F8 (try out which function keys do it for you and how to get back to the GUI). There you can use `ps`, `kill `, `reboot now` or restart the GUI. I'd keep a note of the exact commands on my phone just in case.
As a sidenote, I've had an issue on a good windows desktop PC where it freezes when watching YouTube or ither streaming services. I can still move the mouse for a bit, but clicking does nothing and I'm unable to even save it from a hard reset with Ctrl + Alt + Delete. Sometimes it won't happen for weeks, but if I'm unlucky multiple times in an hour. Look up "PC freezes while watching YouTube" and you will see I'm far from alone with this issue, Asus motherboards with new AMD processors seem to be the common factor. Haven't had this happen on Linux yet (dual boot) and I've tried everything to avoid these unusual freezes in windows, but nothing seems to help.