failed authentication error, help me plz

Cannot access my router, i keep getting the "authentication failed" error message. I am trying to log into the modem/router but I continually get the error "authentical failed".   I have reset it, held for 10, 15 and even an entire minute. I unplugged it, even reset it and still I continue to get the failed authentication error. I am using:   Admin Password   I have an Xfinity Arris TG-862G modem.