Recently I took a chance to install some updates in MacOS X Catalina and after restarting system, all sudden my admin user disappeared. I thought I lost all my data and took me a while to figure it out what really happened.
Actually, after upgrades MacOS x startup disk volume were changed. I don’t know how this happens. If you have multiple volumes to store files & data, there must be a default start up disk. Below snapshot shows there are two volumes and if you are not sure which you have been using before upgrade, try one by one. In every try, you will see login screen after restart, if you identify your user in login screen then that is the volume you should choose as start up disk volume.
How to restore a deleted user’s home folder in Mac® OS X
If you have lost your users and deleted users, below video explains step by step to recover it.