I had a similar problem the other day with a computer on another network. In my case the router access control function was turned on, and did not contain the details of the new computer....... Hopefully you problem is as easy to solve as that !
Incidentally, I don't use the access control function on my own router at this time, as I my router started doing odd things ( like dropping various setting etc ). I put the problem down to either faulty, or insufficient memory in my netgear router. I removed the 15 computers etc. from the access control list, and no more problems.................