I don't know what the cause was but I don't think that a missing driver was the issue though. There were no drivers in the Software & Updates panel.Glad to hear a System Restore helped.
Have a look through the dmesg log it may reveal what's causing the freezing.
When I was new to Linux I didn't know that a driver had to be installed for the GPU and my system would lock up all the time. Not sure if that's your issue or not:-
Installing the driver for your GPU may help. (if you haven't already)
To find out what GPU you have run:
lspci | grep -i VGA