Some combination of things caused my machine to freeze, probably to do with the browser somehow. There were a number of tabs open in Chromium and Firefox, one had a live video stream, another had a CodePen project. The only other time i've it has become mostly unresponsive was when i had a website cross browser testing service open, also through CodePen. That time it recovered when i closed that tab. This time i did a hard restart, because i had no use of the mouse - and i was in a hurry to get my live video stream up again.
The mouse still doesn't work, although everything else is fine. I tried "sudo apt-get install --reinstall xserver-xorg-input-all". I really don't know what i'm doing, but the reinstall seemed to execute correctly. I also have a Wacom tablet connected to the computer and i am using it for now - it works fine (well, as fine as it did before, there are minor issues i haven't gotten around to fixing).
How do i reinstall my mouse?
Update: The mouse is wireless and i assumed it would be detected again after the xorg reinstall. I never thought to switch it off and on again. So, that was silly, and now it is working fine. (And the system is fine.)