Need Phone in morning! Phone start w/antitheft alarm, after enter pw it restarts

To whomever might possibly be able to help:

I am desperate for a fast resolution. About 2hrs ago I was scrolling through my programs menu, when suddenly phone restarted. When it came back on, it gave a message saying “to activate your new phone…” (w/the factory icons, etc only on the screen). Then suddenly “Avast” pops up with this extremely loud siren saying that “Warning this device has been lost or stolen”. I just got password from previous owner, but as soon as enter it, app goes away & goes to the factory home screen, than reboots about 3 seconds later… and keeps on going in the same sequence.

I must have working phone by morning. What do I have to do? Yes, if there is a way to recover &save my stuff, I would prefer to, however I am desperate so if I cant easily then fine… Just tell me exactly what I must do.

Thank you for your time &assistance

this seems not to be anti-theft problem, it seems your OS got corrupt somehow. avast just jumps in because of the corruption it might think that there is sim card problem. imho in such cases a hard reset helps.