Reset my router to get to admin section

Okay I have a modem router combo c7800 $300 my problem is I had someone reset it and I can't figure out what the default username and password is to get into the router configuration. I would look at the Box or the router but I am handicapped and can't get to either. The firmware needs updated so I'm asking if anyone knows the default username and password it is not admin and password. Any help would be greatly appreciated. Thanks in advance.