can't go to administrator

Hi, after a month i tried to go to administrator section but it says "JAuthentication::__construct: Could not load authentication libraries. Username and password do not match"
i cant enter using my username and password.
Is it a general problem or something to do with template.

Sorry, it was a stupid error from my side. i don't remember how it happened but plugin folder was missing. i have resolved by reuploading the folder. thanks anyway