You could probably place a call to Comast directly and ask them to reset it. This way, they'd be able to verify the ownership of your account and reset the gateway's password. You can reach them toll free at 1-800-391-3000.
Welcome Christine. I verified that the default username and password are active on the SMC Gateway. If you are still having issues please send me a PM (private message) with the account number and the RF MAC Address of the SMC.