One cause for this issue can be change in password for account with which Application pool is running for changing the Managed account password follow below blog post:
(Never change it from IIS directly)
If this is not your case then it can be below issue:
If you check the eventvwr logs you will see below entry in it:
The Module DLL ‘C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\isapi\owssvr.dll’ could not be loaded due to a configuration problem. The current configuration only supports loading images built for a x86 processor architecture.
1. Open IIS by typing inetmgr in RUN.
2. Click on Application pools
3. Click on particular Application pool for your site
4. Right Click Advance settings
5. Set Enable 32-bit applications to false