If you have recently edited your reporting services web.config and can no longer access your reports you may want to check if your web.config file contains an xmlns attribute on the configuration element. If it does, you will not be able to access your report manager.  Simply removing the attribute will solve this issue.

I ran into this issue today after i edited a property in the web.config using the IIS tool (under asp.net tab). As i had no idea what had originally caused the issue it took a few hours to figure out that it was because i had edited the web.config with IIS.

Here is the error message that the report manager would display:

"The report server is not responding. Verify that the report server is running and can be accessed from this computer."

Here is the error message from the log file: 

Microsoft.ReportingServices.Diagnostics.Utilities.InternalCatalogExcepti
on: An internal error occurred on the report server. See the error log
for more details. ---> System.NullReferenceException: Object reference
not set to an instance of an object.