*I had autosave switched off
This doesn't sound good, but check anyway, if you have the mymapname
.bak file present at your maps directory. If it's there, just change the .bak to .map and try to load it in Radiant. If that doesn't work, or you don't have the file, then it's time to weep.
You might also check if the .map file size is very small indicating directly there's nothing left of your map. If the file size is normal (quite big, if the map was big) it may be possible to check the .map in Notepad to see, if it contains some very plain error causing a disruption, when you try to open it in Radiant.
This kind of situation occur, if you hit the save button and then, without giving the saving process a proper time to occur, you just start drawing. It has happened to me, and undoubtly to a lot of other people. The .bak file saved me alright. And also I make very regular back-ups of my maps. It's a habit one often learns the hard way.