I haven’t opened Obsidian for a few months. When I opened it ALL my notes have disappeared. Guess I won’t be using Obsidian, unless somehow I can retrieve my notes. 
Hi! Sorry to hear about that.
Obsidian vaults, where notes are stored, are just regular a regular folder (or folders for multiple vaults) on your hard drive. If you can remember the path, the location, where you decided to save the vault, that will go a long way in folks helping out.
It’s possible that folder was in iCloud Drive, OneDrive, etc., and the files were offloaded to the cloud as you haven’t used them lately.
The more information you can share about your setup, the better.
You can also search for a collection of .md files on your hard drive; the folder those are in could be the vault.