After reinstalling Pop-OS on my machine, my login account wasn't visible on the login screen, but it was still fully functional when I entered my username and password. After a good bit of searching, I discovered that the installer had marked the existing accounts as system accounts. Changing them back solved the problem. This is how I made the change.
sudo vim /var/lib/AccountsService/users/<old username>
Change the line that looks like the following
SystemAccount=true
to
SystemAccount=false
Restart the accounts daemon to realize the change.
sudo systemctl restart accounts-daemon.service
When you log out you should see your user on the login screen.