[SOLVED] After update, Laptop won't boot [FAILED to start Light Display Manager]

Ultimately, after trying numerous fixes, I purged lightdm, removed leftovers (autoremove), then installed a different dm (lxdm). Initially, it didn't work because cinnamon was apparently completely gone, so I had to reinstall cinnamon, then got my desktop back minus many critical programs. For instance, I cannot gain access to terminal, have no program manager, firefox is missing, and so on. So it seems I finally resolved one problem but now have several new problems. Do I need to do an update at this point? Do I need to purge then reinstall other things, like xorg, greeters, gnome? As usual, I am clueless about the causes and how to proceed.

The new trouble deserves a new thread with a suitable subject.

Whether purging lightdm and installing lxdm was the proper fix, and whether it was done improperly, is now a moot point. I've got my desktop back, but with new big problems. And as mrmazda said, new problems deserve a new thread. So, here is the new thread:
https://www.linuxquestions.org/quest...65#post6047865
ACTUALLY, SOMEBODY CLOSED THAT THREAD, AND I CAN'T REOPEN IT. SO I WILL ATTEMPT TO RESTART AN IDENTICAL THREAD AND POST THE URL BELOW:
https://www.linuxquestions.org/quest...41#post6048241
Thanks everyone for your help.