Did it stop working all at once, or did you lose the lower fan speeds at first?
Ok, you have a faulty blower control module, which is located underneath the passenger side of the dash. GM has an updated part number, so you will want to buy the part from the dealership: #93803636.
It is fairly easy to replace yourself. Takes maybe 15 minutes. Remove the trim panel underneath right side of dash and look up behind dash and you will see the control module next to the blower motor:
Believe it is around $75 for the part