I have a pair of B&W Matrix 801 S3 with a non play

An open circuit is most likely. Some wire has broken loose from vibration. You would need to take the speaker apart to find the crossover wiring and check it. it might be a blown cap, if no loose wires or bad solder (broken) solder check for a leaking capacitor. (but I bet it is a loose wire)

Replace caps as pairs....do both speakers at the same time with the same cap, if caps turn out to be the problem.
just my opin.

Sand Cast resistors can also crack/break, so inspect them, too.

undo the binding post cup on the back of the spkr, and check the wire attatchments inside, tighten the nut on the back of the binding posts inside. easy enuff to do start there. check your amp too, L&R replace the spkr wires to check.