Error in Web Page

Question

hello guys how r u ??

my question is that, i have created a web application on asp.net 4.0 and i gave my application for testing purpose, the problem came at that time , tester loged in to my application and went for tea break after 10 to 15 min, when he came and he click to menu to go on home page so the error or exception occured like the session time out i dont know i was unable to understand that,

please reply soon


Thanks