So, following is my guideline in solving this problem :
1. First of all we need to check the debug configuration, debug mode is enabled? If its not, please enable it by following the link.2. If there are any syntax error in web.config, please remove those to enable debugging.
3. We need to verify that the Application Pool we are using is started or not. There might be some errors that shutdown the App Pool. How to start Application Pool, follow the link.
4. Further, we needs to verify that App Pool is running under valid account. It may vary for different environments i.e. for my machine, its running under ApplicationPoolIdentity built-in account.
5. Verify and App Pool to appropriate framework version.
6. I have seen that for some scenarios where we are using URLRewrite module, same problem can occur. So, we need to remove or comment the those entries from web.config to make it working.
Hopefully, following these steps will resolve the issue. Please add comments if you have a different solution to this problem. Thanks.