All of a sudden, the session data was lost and gettting "Object reference not set to an instance of an object".

We have an application developed on asp.net. It is running fine for the last one year. Recently we moved the code to another server. It was working fine even after migration.  All of sudden, we lost all the session data and we were getting the below error. 
c0fdb543-36fe-4d50-98a7-a259a9864745 0904b7f2-a4ed-428f-80ef-5282acd43142 TVPageBase - unhandled error: Object reference not set to an instance of an object.
Additional info : System.Web.UI.Control.RenderChildrenInternal(HtmlTextWriter writer, ICollection children) at System.Web.UI.Page.Render(HtmlTextWriter writer) at System.Web.UI.Control.RenderControlInternal(HtmlTextWriter writer, ControlAdapter adapter) at System.Web.UI.Page.ProcessRequestMain(Boolean includeStagesBeforeAsyncPoint, Boolean includeStagesAfterAsyncPoint).
After restarting the app pool, the error vanished and the application is working fine. When we checked event viewer, there were no evidences to say about ...