PTMapper has an option to save a backup copy of your map.
Click Options>Options and check the box which says:
“Keep a backup (*.bak) when opening or saving a map file”
PTMapper will save a backup copy of your file in the same folder as the map file.
To use a backup of your map file, locate the .bak version of the map (e.g Mymap.bak) and rename it to be an .mpd file: