My first question for you would be this... how are you passing the variable?
Are you passing the variable in a session? Cookie? as part of the calling URL?
If a variable is declared in one page of the site and then an attempt is made to call it from another page in the site, unless it's 'passed' somehow then it doesn't exist. Variables are generally 'local' to the page they are created in (similar to how they're local to the form they're created in within a win-app, only you can't just reference the original form to call the variable).
Also, there's a helpful set of references on the msdn site specifically about methods of passing info from page to page in an ASP.Net site.
If, on the other hand, this is not what you need, perhaps a more detailed explanation of your problem coupled with some code snippets to show where the specific issue is would help others find a better solution.
Hope this helps :) Please remember to mark your thread solved once the issue is resolved.