Request for security token failed with exception

Hello All,

We have SP 2010 Enterprise on Windows 2008R2 server.  The server is configured with FBA.  Recently our server stopped allowing users to login, instead it shows a correllation error.

The correllation error states:

Request for security token failed with exception: System.ServiceModel.FaultException`1[System.ServiceModel.ExceptionDetail]: Default Membership Provider could not be found.

An exception occurred when trying to issue security token: Default Membership Provider could not be found. (C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\14\WebServices\SecurityToken\web.config line 98).

I see that the error states that the default membership cannot be found but what is that pointing to?

Thanks for your help