There is a fan control module somewhere under the dash. Can't describe it since there are several types, depending on what Damon installed in your rig. Follow the wires from the control on the dash and you should find it. Odds are it need replacing, but maybe some wires have just come loose.
Most any a/c repair shop or auto electrical shop could fix it for you as well, but labor costs could be high.
Fan controllers typicallyhave one fixed speed and additional ones are
There may be a simpler problem,, I assume the for speeds are low, med-low, med-hi and High and the one speed that works is high, right. The other 3 speeds are not working?
Before you check the fairly expensive control module, locate the resistor and check it, This will be a device with 4 wires goign to it that is attached to one of the air paths, Very likely on the outside of the firewall (But possibly inside) it Upon removal of the device (Detach the 4 wire-connector, remove two screws) you will see a coil of wire, odds are there is a break in one of the wires, Replacements are inexpensive and you already know how to reassemble (reverse of dissassembly) Trick is knowing which device is the resistor... I've seen enough of them to know, but I can not describe it other than to say it's a metal plate about an inch wide by 3-4 inches long, screw at each end, and as noted, 4 wires, one for each speed, One or two of the terminals (usually the end ones) may have two wires hooked to a single connector.
Note if the only working speed is NOT high (IE If it's low) then this part is not bad
There may or may not be a resistor. Most newer rigs will just have a control module that uses a power transistor to change the fan speed. If it does use resistors, they may look as John said, or they may be individual resistors (metal, about 2-3 inches long with a wire coming from each end), or could be the old style which is simply a coil of resistance wire with taps (electrical connections) in several places along its length.
Note: If it is a a resistance wire coil, chances are very good that it will be mounted with it facing the INSIDE of the heater duct/housing through a hole somewhere. If you see a thin, flat, square or rectangular piece of insulating material with several spade connectors and wires connected to them, that's probably it.