Just had the horrible 503 Service Unavailable issue when trying to view a Classic ASP site hosted on Windows 10 IIS. Turns out the Windows 10 Anniversary update broke it... but I digress....
In IIS I saw that the Classic ASP App Pool had stopped. But if restarted, it just stopped again when I hit the website.
Event Viewer said "The Module DLL C:\WINDOWS\system32\inetsrv\rewrite.dll failed to load. The data is the error."
This is the IIS Rewrite 2 module.
So I went into Programs and Features, located "IIS URL Rewrite Module 2" and hit "Repair".
Then I restarted the crashed Classic App Pool in IIS, viewed the website, and the problem was fixed.
If I helped you out today, you can buy me a beer below. Cheers!