I've encountered a macOS bug where a user account will disappear. If there is only one user on the system, then macOS will provide the "Other" option for manually entering the credentials. If you are encountering the same bug, then you will need to trick macOS into running Setup Assistant again so it will create a new admin user account for you. Once another user account is created you should be able to log into the original user account.
To create a new admin user will require you to boot into Recovery Mode and launch the Terminal app from the Utilities menu. Then you need to verify the name of the boot volume and use this name in a command to delete a macOS system file which will force Setup Assistant to run the next time the computer boots. It is possible you may need to temporarily disable System Integrity Protection (SIP) in order to delete the system file.
Disable SIP:
csrutil disable
Review the mounted volumes to confirm the name of the boot volume. The default boot volume is "Macintosh HD" unless you changed it at some point:
mount | grep -i volume
Now delete the hidden system file so macOS will run Setup Assistant the next time the computer boots making sure to replace "Macintosh HD" with the actual name of your boot volume if it has been customized:
rm -i "/Volumes/Macintosh HD/var/db/.AppleSetupDone"
You will prompted to type "y" or "n" for "yes" or "no" to confirm if it is Ok to delete the ".AppleSetupDone" file.
Now re-enable SIP:
csrutil enable
Now reboot the computer and if the ".AppleSetupDone" file was successfully deleted you should be greeted by Setup Assistant. After completing setup you should now be able to access your original user account. You will need to keep this extra user account around until Apple actually fixes this bug.
Your other option is to restore from an earlier backup from before this issue occurred.
