I need help bad in fixing this problem

Question

when i try to login i get this error please help me in fixing the problem anyone.

Specified cast is not valid.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code. 

Exception Details: System.InvalidCastException: Specified cast is not valid.

Source Error: 

An unhandled exception was generated during the execution of the current web request. Information regarding the origin and location of the exception can be identified using the exception stack trace below.

Stack Trace: 

[InvalidCastException: Specified cast is not valid.]
   PCNAdmin.WebForm1.ValidateUser(String userName, String passWord) +380
   PCNAdmin.WebForm1.cmdLogin_ServerClick(Object sender, EventArgs e) +67
   System.Web.UI.HtmlControls.HtmlInputButton.OnServerClick(EventArgs e) +108
   System.Web.UI.HtmlControls.HtmlInputButton.System.Web.UI.IPostBackEventHandler.RaisePostBackEvent(String eventArgument) +57
   System.Web.UI.Page.RaisePostBackEvent(IPostBackEventHandler sourceControl, String eventArgument) +18
   System.Web.UI.Page.RaisePostBackEvent(NameValueCollection postData) +33
   System.Web.UI.Page.ProcessRequestMain() +1292