Check your recent files as well as your file share. There is a chance that your map will still be in one of these, if you had out it on your file share before or if you had played the map recently. Forge sessions count.
For avoiding this in the future, there are a few things you can do. When you go to work on a map, load up a dev map first. This will "clear the cache" as we usually put it, and it will prevent the map save-over glitch. This glitch has been around since Halo Reach and has never been fixed unfortunately. The other thing you can do, and what I do personally, is to always use Save As instead of using Save. This will make a new version of your map each time. I always add a number to the end of my map name and I increase it after each Save As. Ex. Shoe's Map 1.0, Shoe's Map 1.1, Shoe's Map 1.2, etc.... This will leave you with a lot of extra versions of your map, so make sure to get rid of the old versions occasionally. Finally, I would always put the newest version of your map on your File Share. This way it will also be stored by Microsoft's Cloud Servers and you will have a backup.
