If it's not here in the NIVIDIA Control Panel to check....
...then you might check in the Notification Area Icons
settings to make sure that it's not set to be hidden there.
If still not, test to see if you may be able to run the exe file below to turn on the tray icon.
C:\Program Files\NVIDIA Corporation\Display\nvtray.exe
If you still cannot get it back, then you might see if completely uninstalling/reinstalling the NIVIDIA driver may help.
Hope this helps,