Session Is Expired

My session was expired last time when I was using my in-house built system. Now when I log in to the system, it takes me to the same log in page and complaints, "your session was expired". It also happens if I am directly excessing a page by giving the url with the same query string, on which the session was expired last time. If I change the query string values then it's okay.
I even tried with a different user.