On the current version (and some previous versions) of PopOS if you are running on a laptop (I have an Oryx for example) and you try to use SDDM login manager you may find yourself with a black screen instead of a login screen after reboot. Pressing ctrl-alt-f2 to get to a shell you can easily fix the problem by adding some xrandr commands to the sddm setup script. Simply add these two commands to /usr/share/sddm/scripts/Xsetup
xrandr –setprovideroutputsource modsetting NVIDIA-0
Then reboot, or restart the login manager and you should be able to see your sddm login screen.