Solution: This is a tricky question. You need to first figure out what error they are getting as IIS websites throw a number of errors. First thing after identifying that it is not related to application code, you can try to check the following things: 1. Check if the server is down due to some activity like patching or any other issue and try to bring the server up. 2. If the server is up then try to reset the IIS. 3. Next, you can check the bindings. 4. Also check if the SSL certificate has been expired, if yes, then obtain a new cert from a certification authority and renew it on the server. 5. Reboot the server. 6. You can also check TLS values from the registry editor if anything changed recently. as most of the time, it is just a little change in TLS value which causes a number of issues.