PC started having screen glitches after update

The -49 and -50 kernels are flawed, and have been withdrawn. There are graphics issues (possibly only with internal Intel video subsystems).

Boot your computer. At the GRUB menu, select the 2nd choice, Advanced Options (or some such), and select your -48 kernel, or earlier if you don't have -48, and run off of that for now.

Keep up to date with Software Updater and once you see the -51 kernel, or later, then select it at next boot.

Update #1 (for @dspacejs):

Start Synaptic and install these files (search for 5.8.0-48)...

Run:

ls -al /boot

Confirm that initrd.img-5.8.0-48-generic is there.

If it's not there, then run:

sudo update-initramfs -c -k 5.8.0-48-generic

Then:

sudo update-grub
reboot

At the GRUB menu, select Advanced Options, then select the -48 kernel to boot from.