Interop engine Connections may go into an error state - easily seen by the background of the connection becoming red.

You can view this information on the Engines page of the Service Bus web interface.

Especially if the recovery steps do not work, post the details of customer and environment to the Interop Brain.

To fix

First Reset the connection.  Click it once, and refresh the page until the new state is shown.

A successful Reset will move the connection into the Stopped state (yellow background).

Now click Start for the connection. Refresh the page until the new state is shown.  It should resume running and return to the green 'happy' state.