one of SBA server MOH not work suddenly.

Check SBA server event log don't have relate MOH error and main site and some SBA servers MOH is work. 

How to trace the problem? Debugging log?

reboot SBA ?  

Gateway didn't have change recently and MOH suppose is play by SBA server right?

Thanks.

Music on hold is actually played by the clients themselves.  If it's enabled in the client policies:

Get-CSClientPolicy | Set-CSClientPolicy -EnableClientMusicOnHold:$TRUE

It should play, but there are multiple factors that can affect it including client settings and if the endpoints support it.  I'd start by checking the above policy and verify sound settings per Ken's blog: http://ucken.blogspot.com/2010/10/music-on-hold-for-lync-clients.html

Make sure it's working when a Lync client puts a call on hold first, then expand out from there to see if other endpoints aren't working.

Call on hold not working. 

open debugging log and found this error log.

SIP/2.0 491 Gateway side reinvite failed, pass result to Proxy

refer to this GW setting not work.

https://social.technet.microsoft.com/Forums/lync/en-US/9a51f3c3-4cf4-4351-a27b-4c1fda61c11f/lync-hold-not-working-with-incoming-pstn-calls-with-audiocodes-mediant-1000-and-media-bypass?forum=ocsvoice

Hi 

I was helping another forum user the other day with a similar issue. If you are using the SfB client it seems to have an issue playing the default moh from its native location. I found by moving it to another directory or root of c: and changing the client policy to look for the file in that location moh works.

Set-CsClientPolicy -Identity Global -MusicOnHoldAudioFile c:\defaulthold.wma

Try that and see what happens, worked for me.