More information would really be helpful
There are so many possibilites when you say that you can't login. For example, does it say the username and password or wrong, does it go to a blank page, does it just go back to the login page without any error messages, does it.........
get my drift?
I searched through the forums since that problem has occured a couple of times but there are no posted fixes.
Recommendations I found were to try a different browser, clean out all cookies and temporary browserf files, restart server, and contact support via email.
So I would suggest emailing support with all the background infomation (what version your running etc.) and see what they say. Once they get it fixed please post the resolution so people don't have to keep contacting them about this problem.