When We come across this kind of errors ,To know the reason for this error we need to follow the steps.
->Open IIS as Administrator and select web application in which your site is existed
->Right click the web application and select explore
->Now open the web-config file in notepad and type callstack .Set callstack to false if is true else if callstack is true set it as false
->And search for customerror mode and set it to off if it is off else if customerror is off set it as on
