Try using a different browser for the login tab. I usually use Chrome or Firefox. ^^
I've actually been having a few problems with the Login/Play Game Tab and actual In-Game login lately, so we both might be suffering from that. (I've found that it's most likely a problem to occur in the England region, from my own and my friend's experiences, for some reason.)
But, as the others suggested, it's most likely that you should try another browser or clearing your cache and cookies may help.