Server Error in '/' Application. Runtime Error Description: An application error occurred on the server. The current custom error settings for this application prevent the details of the application error from being viewed.

<customErrors mode="Off " />
<SafeMode MaxControls="200" CallStack="true " DirectFileDependencies="10" TotalFileDependencies="50" AllowPageLevelTrace="true ">
< compilation batch = " false " debug = " true " >

Change the web.config file in below locations...............
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\TEMPLATE\LAYOUTS\web.config
C:\inetpub\wwwroot\wss\VirtualDirectories\Port_Number\web.config
C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\CONFIG\web.config
finally reset iis to do go to windows start->run->iisreset  (for this you must have administrative permissions)