.Net 3.5 gridview Problem

Question

I have develped one web application in .Net 2.5.   I have  open that application in 3.5. and i want change the Gridview Format, just i right click on grid go to property --then Auto Format select one another Format & Apply. My application is hanging and not allow me to click anywhere on page as well as any where in .Net Enviorment.

please tell why this is happening.