Nobody uninstalled .NET 4, right?
This may or may not work for you (do not copy/paste any of the CMD prompt lines below... that won;t work for some reason. Just type them in):
1. On the computer that is running Microsoft Dynamics NAV Web Server components, open a command prompt as an administrator as follows:
a. From the Start menu, choose All Programs, and then choose Accessories.
b. Right-click Command Prompt, and then choose Run as administrator.
2. At the command prompt, type the following command to change to the Microsoft.NET\Framework64\v4.0.30319 folder, and then press Enter.
3. cd\Windows\Microsoft.NET\Framework64\v4.0.30319
4. At the command prompt, type the following command, and then press Enter.
5. aspnet_regiis.exe -iru
6. [WAIT... could take a few MINUTES!] At the command prompt, type the following command, and then press Enter.
iisreset