Administrator account apparently lost after forced reboot of OS X Yesterday I ran into one of those rare problems with OS X (10.11.4) where I thought I had to reboot my MacBook. The cursor wheel was constantly spinning and I was unable to quit any apps or even access Force Quit .... (This was triggered inside Firefox.) So a reboot seemed like an easy way out.

Unfortunately the reboot seems to have corrupted the setup in a bad way. I normally have two user accounts on the MacBook, like A and B, where A is also the administrator account. 

After the reboot A now seems to have disappeared. It e.g. does not show up under System Preferences | Users, nor in /etc/passwd. It is also no longer available as a choice in Login Window .... The lost account's files still seem to remain under /Users/A/, but I can no longer access them because I no longer have administrator privileges. 

I this a know problem (perhaps even an indication of malware)? How can I restore account A and its administrator privileges without risking corruption of its (presumably) remaining original files in /Users/A/? (At least I do have recent Time Machine backups and can log in as B.)
Turns out I was lucky. Booting into Single User Mode (for obtaining administrator privileges as described here) apparently caused some extra disk check. This presumably led to the restoration of some corrupted piece of (meta)data and in effect my account A, including its previous administrator privileges. 

Looks as if the system is back to normal now: much relief!