Here are a few questions that will help us find, duplicate, and hopefully resolve the problem:
Did you receive any error messages (if yes, what were they)?
What exactly did you do before / after the problem happened?
How long has it been happening? If only one time, what day and time did it happen?
Which browser and version are you using (eg., Firefox 3.5.9, Internet Explorer 7, etc.)
Have you logged out, deleted your internet cookies, logged back in, and tried again?
Have you tried a different browser?
Have you tried a different computer?