Alright, well when you attempt to log in, and the screen just refreshes, there should be a message on the screen in red explaining why the log in failed (it's sometimes hard to see but it should be there). It's usually because your password was filled in incorrectly, but there may be other reasons, such as SE wants you to change your password on their billing web site due to what they call "suspicious log in activity." The red message should have the explanation you're looking for as to why you can't log in.
Edited, Mar 11th 2012 3:14pm by Xoie