Some plugins may be moved when installing a ProTools update. Check for plugins and disabled plugins by going to:

In Mac OSX:
Macintosh HD > Library > Application Support > Digidesign
This Digidesign folder will have "Plug-ins" folder and a "Plug-Ins (Unused)" folder.

In Windows XP:
C:Program Files > Common Files > Digidesign > DAE
This DAE folder will have "Plug-ins" folder and a "Plug-Ins (Unused)" folder.

On both platforms, look inside the "Plug-Ins (Unused) folder" for the item you are missing. If you locate it there, move it to the "Plug-Ins" folder. You'll need to Quit/Exit Pro Tools and start it again to see if it works again.