With the additional input that flicking the switch over and then back to its original position cause the light to turn on -- Are these old-style fluorescent lights? If so you may want to try a new starter or new tube, since either can wear out over time and start wanting an additional inductive "kick" to help them get going.
But if up used to be on, and down is now on, and nobody had modified the wiring, that's almost gotta be a three-way circuit. Or four-way, with one or more DPDT switches in the middle of the travelers. Unless these are smart switches and someone has changed the logic coupling them to the controlled circuit, or someone has physically reversed a switch...