First thing to do is to confirm that there are no breaks of the coil windings inside the pickup itself, this can be done by using a multimeter to read the DC resistance of the pickup, if you get a reading of OL on the highest resistance range of your multimeter then that indicates that the coil-windings have gone open-circuit, also check your earth connections as well, it could also be a dirty contact in the pickup selector switch, in that case do a continuity test with your multimeter, hope that helps.