Hmm, well, what ye could do-although this may change the appearance of y'er map and what is inside it, is revert the file back to an earlier version. {Make sure to save a copy of the file b'fore doing so!} To do that, right click the terrain.cgf that is acting up and select "Revert to earlier version", or something like that. A tab will pop up, giving ye all the versions of that file it has. Try selecting the previous version. Then, head into the map maker and see if it works.
Now, ye may lose a few meshes tha ye 'ave placed lately, but it may load the file and ye can get back to work.
As f'er any other options, ye may 'ave to wait it out. This could be just some weird glitch and everything is fine. It could also be the file has become corrupted. As mentioned by Hugrf, 'ave ye updated the file in any way other than saving y'er work in map maker? This could be the reason as to why 'tis not loading.