My 1995 S-10 Blazer did the exact same thing. The dealer reset the computer
by disconnecting the negative terminal of the battery for a few seconds (and
of course charged me 45 bucks!) His explanation was that if you cycle
through the different modes on the switches too quickly, you can lock up the
computer for the function. I think I caused mine when I was reaching for
the 4 Hi switch and hit a bump in the road. It caused my finger to "double
bang" the switch. After the reset, no problems were noted. Hopefully that
is all it is.