signed out of itunes on pc and unable to sign back in

last night it asked me for my password when i opened itunes first time it came back again(this has happened before usually works fine on second go) but it came back a third time and then i got some error message and now i'm signed out and cant sign back in sometimes i get an error message but other times it seems like its signed in(no error messages) but when i click on account it still has the sign in option I've tried restarting my pc and uninstalling & reinstalling itunes to no avail as a result of not being signed in I cant buy anything from the itunes store nor can i download any new songs on my pc I can still buy and download new songs on my ipad just not on my pc please help