Bane of my life too. My Generic City map suffered the same fate during development. Not sure what I did to cause every material to vanish even to this day! The last thing I did was import a model, apply normal maps and tweaked the textures in the editor.(Just like all the other times) The next time I loaded the map every material was gone apart from the last building I imported. I looked in the material.cs file and all the entries in there were gone apart from the last model. For some reason it deleted everything in that .cs file and replaced it with the last model I imported instead of adding it to the bottom like every other one I imported. To be honest I deleted the whole map and started over, but I have since figured out how to fix it. Its a pain in the arse though. In the shape editor there is a tick box that says Force.dae. Tick that then leave the shape editor. Select a different model with missing materials and go back in the shape editor. Now you should see the import settings box like when you first imported the model. Tick the "Fore update materials.cs" box and hit ok. Hopefully that fixes it next time you load the map. If not, like in a few of my models, it turns into a case looking through the materials.cs file to see if your texture is listed in there. If so then you have to make sure these sections match each other. For me it said mat1 in the editor and not what it said in the materials.cs file. Just make sure to hit enter after changing the material name in the editor, and hit the save button. Hope this helps, if not then I have no clue.
