To elaborate somewhat. The blower resistor probably has a thermal fuse on it. If it gets too hot, it blows, and the resistor no longer functions. High blower does not use the resistors so it is the only position that works. The reason the thermal fuse blew was either your blower motor is drawing too much amperage (meaning it is failing) or that you frequently have it on the lowest speed then turn the vehicle off, and when you restart it with the blower on lowest speed, it gets way too hot and blows the thermal fuse. This happens because the resistor pack depends on air from the blower to cool it, and during the start it sees lower voltage, thus higher amperage and the resistors, are all in series in the low speed position so the most heat is being generated, conbined with the blower not really flowing any air to speak of.
Always turn off the blower when you shut the vehicle down.
I had this happen twice on a '99 Plymouth Breeze, it was annoying to spend $20 to buy a new resistor every time, and I finally figured out which coil of the resistors was the one for low speed (along with all the others) and cut out that coil, then the blower worked on 2, 3, 4 but not 1.
'03 Ram 2500 CTD, 5.9HO six speed std cab long bed Leer top and 2008 Bigfoot 25B21RB.. Previous, Being repaired and will be sold, is a 2008 Thor/Dutchman Freedom Spirit 180. SOLD - 2007 Winnebago View 23H Motorhome.