Why does my power cut off and the engine still run?

I have a 2004 cord explorer sport trac, when I
am going down the road the power will shut
off it still runs but it is like the battery is
unhooked the only thing that works is the tac
and when it comes back on it says test where
it usually shows my mileage, it’s been doing it
for awhile, any help would be greatly
appreciated.