There's a simple fix for this!
Go into your Black Ops 3 Directory and enter Radiant > Autosavebackup, there you will find heaps of auto backups of your maps/prefabs.
In your File Browser, Right click > Sort by > Date.
You want to copy the top one of your Map.
Now, go back to your Black Ops 3 Directory and goto Map_source > ZM.
Paste your .map that you copied from your backups and remove the extra stuff at the end so it's back down to zm_usermap.map
Now open your map in Radiant!
If this doesn't work, go back to your backups and get the 2nd top one, keep going down the list until you find one that works.
What is the meaning of life? Dank Memes.