Startup problem

[ulver]

Well, for no apparent reason, when I start my iMac and fill in as always my password in the login screen, a new login screen appears. Only this time my user account and password are not accepted. Same happens in Safe Mode, while Apple Diagnostics finds no hardware issue.

Please help. Any ideas?​

 

[Fishrrman]

Take pictures and post them here, so we can see what you're seeing...

 

[ulver]

thank you for the answer Fishrrman, but there is no need for screenshots now as the problem has been solved. or, more correctly, the problem cannot be solved because apparently something (no clue what) caused the admin account to lock itself. however, using the rm "/Volumes/Macintosh HD/var/db/.AppleSetupDone" command on Recovery Mode, I managed to create a new admin account with the same name in order to access my data.

currently I'm backing everything up with TimeMachine and later I will reinstall the OS

 

[Fishrrman]

"however, using the rm "/Volumes/Macintosh HD/var/db/.AppleSetupDone" command on Recovery Mode, I managed to create a new admin account with the same name in order to access my data."

I'm wondering if this isn't going to give you permissions problems with the data from old (and now deleted) account...?
Just having "the same username as before" isn't going to solve that...