After reloading I got two strange things :
The already explored mountains, hills etc. wheren't on the map. I could see them again if I explored near them, so they didn't really disapear, it's just a "graphical" problem.
When I loaded my game my kingdom borders weren't there : for instance I couldn't use the "teleport hero to friendly territory" in my territory. Clicking the "next turn" button fixed that.
