95 chevy Z-71 K1500 ac/heater blower won't work

Any ideas as to why a ac/heater blower would quit...........a/c compressor is coming on but no blower function when you try to switch from low to high blower..........feel cold air coming from the lower floor vent but upper vents won't blow...........worked fine yesterday morning, went to town later in the afternoon and no blower...........fuses check ok............MRT