tty & fault finding

I commented on a youtube video about a system locking up and being unrecoverable and someone came back with using tty3 to find out whats causing problems.

My Kubuntu 16.04 Desktop locks up and sometimes I can move the mouse but not select anything and other times the computer completley locks up where I can’t do anything other than force a reboot.

When in tty, what commands do I use to find out what’s wrong - what’s causing it to lock up?

[EDIT]
Computer has now completley locked up since coming out of tty and getting back to the desktop, the mouse pointer is nowhere to be seen and no amount of moving the mouse will make it visible again. How do I get out of this lock up of my system?