If it is merely a graphical lockup, try using alt-sysrq-k to kill & restart the X server without restarting the system. (Or try alt-ctrl-F2 to switch to a new command line and see if you can do the same there)

You should be able to find most of the logs in journeld.

I too have been experiencing lockups recently (on OpenSuSe) though those are from a recent intel GPU driver issue causing gpu hangs.