If you are using PeerBlocker or some such, you might find that it blocks access to some sites. Try closing it to test, if it is then when you connect with it running you can usually recognize the IP that is being blocking and add it to the do not block list. You may also need to update Java or some other add-on. Sometimes sites will check the Java version if it uses it and prevent access if it is out dated (BitHumen did that recently). Other than that, clear your cookies and try again. If you still can't get on then you might have been temporarily locked out, you then gotta go chat in IRC to try to get your account set back up properly (worth a try, but most times it is tough to talk your way back in).