I'm afraid that what you've posted above provides no useful information at all that would help us suggest solutions to your problem.
Please read this link in order to understand the type of information we need in order to help you. Then provide it in your next post.
Of particular interest would be:
1-The name of the JSP that you were requesting when the error occurred along with some code snippets from that JSP. 2-Do you have any guesses as to what went wrong? 3-Is this a new application, or is it an old one that was working before you made changes to it? If it's the latter, what changes did you make to it? 4-The portions of your struts-config.xml file that are relevant to the action the was running when the error occurred. 5-A more complete stack trace