Question for AC repair gurus

Last week we turned the heater on, it ran well but after a while, we started to smell burnt plastic. Turned the unit off, went about out day & kinda just forgot about it because there was no need to run it. This weekend we turned the AC unit on but nothing happened? Switched to heater, still nothing. Even put the unit on [ON] but it would not come on. The unit outside not the unit inside would turn on. Any ideas of what may or may not be the problem? Unit is old but my repair guy said it was in good shape for it's age.