Any save I had under 100Mb was not damaged by whatever did this. Also my map was an Xbox 360 map originally but, I have a map that was started on the 360 that came out undamaged. Previous to this event I haven't had any issues with this map so I can't really link it to anything. Thanks for the speedy response and I hope we figure this out.
I am having the same issue I logged in today and my map was renamed JQEAADzxAQA= and was reduced to 0.01MB I really don't want to lose this map I have been building it since the Xbox 360!
My world has been restored, I had to select the file that had been corrupted and then the maps resynched and my original map was fixed. So happy it is finally fixed.