Mine is still gone and I've tried everything.  I opened a ticket with Cakewalk support but they won't tell me exactly how the Insert Synth menu is filled out, if I knew what it was doing I could figure out how to get it the data it needs.  Their only unhelpful advice was to uninstall everything, delete every directory and reinstall.  Which is something I'm not willing to do for a feature I rarely use (I can insert synths using the Plugins panel just fine).  If they would just tell me how exactly Sonar fills out that menu, I'm pretty sure I could fix it for myself.
 

Hi msorrels
 
i have noticed a file that appears in 2 folders - c/cakewalk/shared utilities and also c/app data /roaming/cakewalk/sonar platinum/plug in layouts  the file is a text document
                         (RIGHT CLICK THESE 2 IMAGES ABOVE AND CLICK VIEW )
i have also opened a ticket with cakewalk and supplied them these details , they must have something to do with it because this file appears in the 2 folder locations where the files for plug ins are kept.
 
Any ways >

IMPORTANT UPDATE
 
I have managed to find another fix > yes a different one again > no consistency on any of the fixes so far .
 
I have not done a re scan yet since this fix but when i buy a new effect or instrument i will then ,as i cant  afford to spend anymore time on this issue at the moment .(Although Í am very tempted as i think this is THE FIX until cakewalk sort it.
 
(DISCLAIMER ***please make sure you have a copy of your custom PLUG IN PRESET/FILE the .pgl ones you made in plug in manager , before you do this , though when i did it it did not delete any of my custom plug in presets/files .pgl)) just to be safe .
 
( ALSO MY - INSERT SYNTH_ WAS COMPLETELY GREYED OUT WHEN I DID THIS - AND THIS IS THE MOST PROMISING FIX SO FAR FOR ME  )
 
So the fix is : please follow exactly . Good Luck
1 )Open sonar  > Edit> Vst settings
Now under> scan options click reset >then yes to the pop up window
( Also please read the pop up window before clicking yes- though its safe to do)
 
if you don't have custom preset /plug in /.pgl  folder/file  this may be all you need to do ))
((Just click apply and close preferences and see if insert synth is back ))
 
however I have a custom folder for instruments and effects so lets press on.
 
2) Now under scan options still >
  A) change manual scan to scan on start up.
Then check / select
 A)rescan failed plug ins and
 B)rescan existing plug ins
 
Next  shut sonar platinum down > then started it up.( No need to do a PC restart )
 
Ok , naturally on start up the plug in scan starts too , so the first thing i checked was insert synth and it was back (not greyed out )however - as the scan was going along so to was my custom files appearing under insert synth , so be patient and wait for the scan to finish  .
 
Ok now the scan is finished your insert synth should be working and have all your custom vst list  back
 
3) Once scan is finished > go to edit > preferences >Vst Settings
A) Uncheck Re scan failed plug ins
B) Uncheck Re scan existing plug ins
C Change back to Manual scan
 
click apply and close .
 
All done
 
Remember i have done  of different fixes for this problem and am not sure if this will work for anyone else , though i will share any fixes in a hope it helps one other person .
 
Also AGAIN i have NOT done a re-scan after these steps  YET > though i am very curious to find out once insert synth greys out again if this fix will be the light at the end of the tunnel .
 
God Luck God Bless
 

Resetting and redoing the VST plugin scans has no effect on my Insert Synth menu.  Nor does any changes to the various plugin layout files.  I've deleted them and recreated new smaller ones in many combinations to no effect.
 

Not sure if this can help but worth a try.

I always do a c drive search for a file named ring_dp_dump.log

If it's there it will be in a sonar directory

I always delete this file each time before I spend time trying to fix this .

Maybe you should give it a go first . That file is defiantly generated by the scan because I have had explorer opened when it appeared during a scan in the plug in folder which prompted me to do a c drive scan and then I found another in shared utilities . So now I delete them .

However that fix has worked for me so until I do another scan , I will let you know what happens then .

I can't do a scan until I really have to because this is a serious pain in the a--- , and very time consuming to get inert synth back each time , and I need it working , because when it goes grey , I lose my custom layout even in plug in menu or main menu .

Good luck

I did a full scan of both my C: drive and my D: drive (where plugins live) there is no ring_dp_dump.log from the name of it I'd suspect it's being created by one of your plugins.  That string also doesn't appear in any of the Cakewalk utility programs.  I think it's a red herring.

sorry to hear that , though it doesn't surprise me , every time i have got insert synth back (besides doing full installs) has been a different solution , so it amazes me how it is possible for all these solutions to not be consistent each time , like a disease building resilience each time (: 
 
Anyway all i can do at my end is post my fixes and hope it helps someone , also i will post as soon as i do a new  re scan and save a new instrument into my  instruments.pgl .
 
you know i also have a effects.pgl and it has never caused an issue , even when insert synth is greyed out and i lose my custom instruments.pgl EVERYWHERE , effects still works ,all my custom effects have never gone away .!
 
All the best from down under