I had an issue with a soft synth, where all of a sudden the sound cut out and now the track in question is silent.  I've deleting and re-doing the track, but it's not working.  This only seems to affect soft synth using Soundfonts, as others (such as Cakewalk Sound Center and Addictive Drums) still work fine.
 
Does anybody know what could have caused this and how to fix it?

Could this be the classic "Increase the number of Midi Buffers" problem?
 
Go to:
Preferences>Midi/Recording and Playback: Numer of Buffers
 
Shouldn't be an issue in SPlat anymore, but who knows...

jpetersen
Could this be the classic "Increase the number of Midi Buffers" problem?
 
Go to:
Preferences>Midi/Recording and Playback: Numer of Buffers
 
Shouldn't be an issue in SPlat anymore, but who knows...

Thanks I'll check that out.  I'm using Professional so it may well still be an issue.

Typo alert:
I meant "Prepare using xxx milisecond buffers"
I had a look: Standard seems to be 500ms, I have mine increased to 1000ms.

Sound fonts? I haven't heard that word for a long time...Are you using the on board sounds? 
Seems all the soft synths I'm using use Samples,, sound fonts were like 1998. but then..

Soundfonts are a combination of a text-based SFZ (or SF2) file and sample library; the text file contains opcodes that tell the player how to use the samples and respond to controllers. Dimension, Rapture and Session Drummer are all SFZ-based; it's still a totally viable soft synth technology. I use the old SFZ player and the free Sforzando player regularly. They work just fine in Platinum.
 
I would second the recommendation to raise the Prepare Using Buffer. Since X1, the default has been 250ms which is too low in a lot of cases. The old default of 500ms should do the trick.

It might be a channel assignment, that messes things up. You can use Sonar's buit-in channel assignment to fix a particular MIDI channel. Also check your MIDI object in Event list editor. Maybe some CC mutes data output.

Thanks all, I will have a play with this and see how it pans out.  And yes, Soundfonts are a nice and versatile way of increasing the soft synth library.  There's a lot of good free ones out there.