iMac System Lockout

I have a 27” iMac running most recent updates. Yesterday when I first logged in my password would not work to open the system. After a few tries, it asked for File Vault Recovery. I entered code, then asked for new password, I put one in then at the bottom it had a section to put in your password, so I entered the new password, and nothing happen (still in reset password screen) so I tried old password and the computer gave me access to my files etc. well that was weird so I logged out, then logged back in using old password. System is fine all day. This morning I get up, old password doesn’t work, try to enter new system password, and still locked out. Was my system hacked, or what is this? I need help, work from home and all my files are here and hard to work from iPhone. Anyone out there who can help me get through this hurdle?