It suddenly appears after update to windows 8.1 Operating system or when deploy and application to the cloud
Reasons/Solutions:
1. Maybe in the Windows Application log was the following error;
The Module DLL C:\WINDOWS\system32\inetsrv\rewrite.dll failed to load. The data is the error.
In order to solve you could:
a. Reinstall the Url Rewrite
b. Restar IIS
2. When .NET application is deploy to cloud, and the IIS Version property is set to IIS 6 or lower, after deployment the application doesn't work and gives this error: 503 Service unavailable.
In order to solve you should change the IIS property value, and deploy again (F5)