Cool/warm air knob broke off/isn't working?

If the entire knob came off you should be able to see the metal switch. From there all you need are some pliers and for now turn it that way until you can get the real knob. Just use the pliers to turn it from cold to hot. The knobs can be found either at a junk yard(ask first to see if they have your car year and make), dealership, and on the Internet. Best of luck!