From what you posted it sounds as if you are connected to the network since you have browsing ability which limits your issue down to software. Logging into a website is mostly a software access function and not a connection function. This does not sound like a router issue as you would either not be able to log into a particular site at all or have full access however have you modified your router settings in any way? You can put the router back to default settings by holding down the red reset button for 10 seconds with it powered on. Also, are you having an issue wireless and if so have you tried wired for testing to rule out encryption issues?