Tony, Ray pointed out some good suggestions. As he mentioned, there are different situation that causes this to happen. From my experience, this happen when the AMS services did not shutdown properly or someone or application stop one of AMS services to the AMS server plus.
I would first try to use the AMS "Terminate Server" and properly shutdown all of AMS stations. Before you restart AMS client, make sure you restart AMS Server Plus first by manually launching the AMS app from the start menu then now go to AMS Client station and manually launch the app as well.
What this will do is to get all the AMS services to startup properly and get all the connection to the client to the AMS Server Plus establish.
Regards,
Tinh