It's hard to tell you anything without more info. Kinda like going into the doctor and saying "My foot hurts, what's wrong" and he has to make a diagnosis without checking you out a bit more.
Here are a few questions that will help us find, duplicate, and hopefully resolve the problem:
• What is the URL of the page that you got the error or had problems?
• What were the exact steps you took before having the problem? Please be as specific as possible so we can do exactly what you did.
• 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 cache, logged back in, and tried again?
• Have you tried a different browser?