Gee, maybe you're using the wrong password? We can't read your mind. If this is one of the Xfinity Wireless Gateways, then the default userid is "admin" and the default password is "password". If it's been changed and you don't know what it is, then call the gateway support folks at 800-363-2416 and have them change it for you so you can get in, then change to your admin and wireless passwords to your own values and put them someplae safe, just in case you forget, which is real easy to do if you don't use them all the time.
If you have some other router or gateway, we need specifics in order to help.
