500 – Internal server error from IIS on Windows server

Open a web.config file in root folder & add this line  <httpErrors errorMode="Detailed" />

Ex:
 <system.webServer>
 ..................................... 
// comment below lines
 <!--<httpErrors>-->
      <!--<remove statusCode="404" subStatusCode="-1" />-->
      <!--<error statusCode="404" prefixLanguageFilePath="" path="/index.php" responseMode="ExecuteURL" />-->
 <!--</httpErrors>-->
........................................

 // Add this line 
 <httpErrors errorMode="Detailed" />
</system.webServer>