Every once in a while I won’t be able to login to a site and might assume that there is something wrong with my cable modem or router. Before you panic, it could be that just this one particular site is down. There are a couple of different solutions that will provide an answer.
Also before you go out and buy a new router, call your internet provider and see if there are any outages in your area and have them call you when they have fixed the problem. This happened to me just yesterday.