A: It is possible sometimes that the file containing information about the materials used in your model is corrupted. This file has the extension .Mat and it is stored in the same place as the main file of your room, i.e. the .Par file.
In the same place there is also a file with .Mbk extension. This is the backup of the .Mat file and can be used instead of the corrupted .Mat file. In order to do that, you have to erase the .Mat file and change the extension of the .Mbk file into .Mat.