Take the opportunity to review the document at the following link, discuss various aspects regarding the deployment and configuration of MoH:-
http://www.cisco.com/en/US/products/sw/voicesw/ps556/products_implementation_design_guide_chapter09186a0080637422.html
If MoH was previously working verify the following:-
Ensure that the run flag is enable for the MoH sever.
Ensure that the media streaming service is activated and is running. Start if it has stopped, or try restarting it.
Ensure that the MGRL for the IP Phone or Device Pool for the IP Phone has the MoH resource as memeber of the MRG in the MRGL.
Hope this helps
Regards
Allan.