Is this loading a saved game or starting an entirely new map?
If you are loading a saved game, then possibly, your map cannot handle being saved and loaded. There are some other maps like this, although I don't know what causes the inability to be saved and loaded. Another possibility is that the save file was moved, or you no longer have the original copy of the map you saved it on.
For example, you saved on the map Test Map 1.00.w3x. You later opened the map and worked on it, then you saved and renamed it Test Map 1.01.w3x. Since you no longer have the copy of the map the save file was saved on, it will no longer load properly.
As for any other reason, I don't know what else could have caused it.
