I had this problem before. I tried many fidderent things, I don't know what worked though. xD First I tried exporting the map. Then I tried exporting the objects. Then I restarted my computer and poof they came back. xD So... I don't think they were in that order, but once you do one of them restart your computer. If that doesn't work save the file as a different name. I don't think that'll be help but it's something. x3