Have you tried clearing your cache and cookies? that typically helps a lot with these sort of things. Especially if there are other people who are not experiencing this problem. If there are others in your server who also can not login, then I would suggest sending in a Customer Support ticket.

I would suggest starting with the cache and cookies though, and if you need help finding out how to do that on your web browser, then I suggest googling it, or if you are using the client, you just have to click on the gear icon on the top, and click "Clean Up System"

If your still experiencing this issue then let us know and we can look at other solutions =)