pi@raspberrypi:/etc/.pihole $ pihole -up
[i] Checking for updates...
fatal: HEAD does not point to a branch
Error: Remote revision could not be obtained, please contact Pi-hole Support
Additional debugging output:
HEAD detached at v3.2.1
nothing to commit, working directory clean
That will reset the web admin local repository to an exact clone, as long as you haven't made any customization changes then that should get you back to working. Something pulled you out of sync. With the updated version of dnsmasq on your system you should be safe to set things back to how they should be.