500 - Internal Server Error

[DamianLe]

Our webserver has been running with no problems for at least 2 months until yesterday. All visitors to our chat and forums get a 500 - Internal Server Error
Accessing the chat/index.php for the local computer yields the following detailed error:
HTTP Error 500.0 - Internal Server Error
The page cannot be displayed because an internal server error has occurred.
Detailed Error Information
Module - FastCgiModule
Notification - ExecuteRequestHandler
Handler - PHP_via_FastCGI
Error Code - 0x00000000
Requested URL - http://localhost:80/chat/index.php
Physical Path - C:inetpubwwwrootchatindex.php
Logon Method - Anonymous
Logon User - Anonymous

I restored a backup to the server and it works successfully except there are lots of changes that we would have to apply. Another website gave me the idea of testing php using a command line statement. PHP displayed the phpinfo as it is supposed to.
I also gave the folder Full Control in order to test permissions with no difference.

Please help!