Low and medium fan speed no longer works on our Hampton Bay fan when you pull the chain. The high speed works. Can this be fixed with a repair part?

The speed is controlled by a capacitor bank ... high speed uses no capacitors, low speed uses a small capacitor in series with the motor and medium speed uses a larger capacitor ... the pull chain switch selects which capacitor is used ... so if the capacitors go bad or if the switch fails to select them then you will not have low and medium speed ... so you could have a bad speed capacitor block or a bad switch or wiring that has come loose inside the fan (that connects the capacitors) .. all are replaceable .. i would first remove the covers from the fan and check all wire connections .. check the switch to see if it operates smoothly .. then suspect the capacitor block .. if you were an electrician then you could bypass the switch to test its function but if you are not then make sure power is off while working on the fan .. you should be able to get a new capacitor block from the manufacturer .. most are similar if they will phisically fit in the fan housing and have the same connectors ..