Ensure that the Music on Hold (MoH) server has connectivity to the held device.
If Media Resource Groups are being used, ensure that the MoH server is part of the Media Resource Group and that the Group belongs to a Media Resource List. Also check that your Cisco IP Phone belongs to the Media Resource List.
Ensure the IP Voice Media Streaming Application is running and then stop and start the service.
Check the Run Flag under the MoH Server Configuration page, and ensure it is set to Yes.
Check PerfMon or the Real-Time Monitoring Tool to see if streams are available.
If multicast MoH is being used, verify that the multicast stream is making it to the network of the endpoint that needs to hear it. It could be that the infrastructure devices, such as routers and switches, are no longer forwarding this multicast stream. A sniffer is the easiest way to verify that the multicast stream has made it to the Cisco IP Phone's subnet. Also, if it is a gateway, ensure that it is multicast-capable for voice.