How to ensure disposal of MongoDB connection (DB Hanging)
1) It sounds like you are instantiating and using the Repository in the Service class. So instead of
var _mongoRepository = new MongoRepository(..);_mongoRepository.Single(...);you would have
using (var _mongoRepository = new MongoRepository(..)){ _mongoRepository.Single(..);}If you are wrapping Mongo calls in the MongoRepository class then make sure that the Dispose method properly cleans up the _server connection (close/disconnect/whatever). It looks like you are creating the connection in the constructor, using it in the method but never closing it.
2) From my experience it is preferable to open a DB connection as late as possible and close it as early as possible - rather than opening it and using it several times for different queries.
There's a good discussion on MongoDB (C#) connections at .NET best practices for MongoDB connections?