Cracked solder joint on the printed circuit board. All of them do it sooner or later. You'll probably hit a bump at some point and it'll come back on. It's still counting miles, just not displaying them. The solution is to get to the back of the display and re-flow the solder with a heat gun or soldering iron. Usually not worth the trouble though, it will come on sooner or later. Outside temperature and length of driving time can both affect it.