The usual problem is that all speed BUT high work. That's usually the resistor pack that's mounted to the heater box behind the glove box. But I don't think that's your problem.
I'd check the fuses, then the relay if there is one (some had them, some didn't). If it does have a relay, it will be external to the main box.
It could also be a switch. That would be my best guess after you check the other simple stuff.