Sounds like the blower motor resistor has crapped out. Sometimes, one section of the resistor will open up, causing one of the speeds to quit working. Eventually, the thing developes opens elsewhere, and all blower speeds stop working. I'd replace it first, if you're sure the fuse is good. In the future, if one of the speeds quits working, don't wait, but change out the resistor right away.

1977 CJ-7, fiberglass body, AMC 360 w/ headers, DUI ignition, Edelbrock intake and Holley 4150 carb, TF999, Dana 300, 4.56 gears lockers, York air comp, 4" susp lift, 2" body lift, BFG 35" M-T tires, Megashifter, AGR pump & box, REP8000 winch.