This happens all the time. Upgrades don't intentionally delete any user data, but when you have corruption, upgrade time is when major changes cause you to rewrite data on disk and loss is checked or noticed.
You can restore from a backup or make a backup of what is now on the disk and then try an erase install. If you are missing something as large as a user account, who knows what else is missing and unstable. Best to wipe and start cleanly after making a backup.
Also, I wouldn't try to fix anything - trying to make a new admin account could lose you even more data. Shut down that Mac and see about your options for making a backup to another computer or at least get a quote from someone that has the skills to help you back up your Mac.
The first skill to master if you want to try this is booting to target disk mode:
In your case, target disk works when your OS isn't able to give you admin rights. You can skip that if you have a good backup and just erase install your Mac.