You should Google it "fb hacked account" to see what is suggested by Facebook Help Center . If you restore your fb account ,I'd advise to set 2 factor authentication . If you want make a factory reset to your device ,first you need disable the 2FA .And also ,you can get backup codes just in case . Search "fb backup codes" you 'll see the relevant article .
I'm having the same problem. Mine was hacked a month ago. I still haven't got it resolved.
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