I had to reset my iMac to previous date. Now when i go into LR and look ar collections that are synced with my mobile device some on the collections on the desktoop are empty. They are on the iPad but are not on the desktop. How do I get then to sync back to the desktop from the iPad
Have you tried to disable the collections from sync and re-fill those on Lr desktop and start the sync again.
If that won't work I would recommend to trigger "Delete Data" via Lightroom Preferences -> Lightroom Mobile. With that you will start from zero.
Hope that helps.