You'll get better help by describing what you have done, and what the results are, for starters. Identifying your browser and operating system might help as well. (There could be a known issue)
Example;
I have logged in before, but now I can't
I enter my handle and password
The screen reloads the log-in page
OS: Windows Vista Home
Browser; IE-7
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Warning this is strictly Orions McPhees Opinion, it may or may not have any bearing on events true or imagined