Ahhhhhh common issue, I've had it plenty of times. Never been able to "recover" the old .cfg, you will have to re-do the sizing and everything all over again in the map making when you get it back open.
copy and paste this code into your "MapName.CFG" file, and it should make it accessible again.
PageSource=Heightmap
Heightmap.image=defaultTerrain.png
PageSize=513
TileSize=65
MaxPixelError=3
PageWorldX=5000
PageWorldZ=5000
MaxHeight=300
MaxMipMapLevel=5
VertexProgramMorph=yes
LODMorphStart=0.2
CustomMaterialName=Terrain/Default
If that doesn't seem to work, since you have all your objects in your objects file, I recommend making a new map and copy and pasting all the objects in said file into the new MapNameObjects.cfg file in the new map and it should put them all in the previous spot before.
((( It should work tho, and I don't 100% recommend doing this, it should work out but you will have to resize the new map to match the old map as best as possible or all the meshes/objects will be in wonky places.)))
I recommend backing up ALL .CFG files in a different folder NON RELATED to FeralHeart, as things can get lost often.
I hope this helps, good luck with your map!