You've got to track problems like this down by process of elimation.
Feed the subwoofer output from the pre-pro to the channel of your amp that you have connected to one of your mains. Do this using the the regular interconnect from your mains, not the one that you would normally use to connect to the sub.
Obviously, this means that one of your mains would be temporarily disabled, but this would allow you to see if the pre-pro is actually outputing a signal from the subwoofer output. If it is, you should hear the deep bass coming out of the one main that you selected. If you get nothing out of the main, even after fiddling with the pertinent subwoofer output level controls and adjustments, the pre-pro has a problem with it.
If you hook things back up normally and you still get nothing out of your subwoofer, your cable going from pre-pro to subwoofer is bad. It is possible that you had a poor connection causing the hum and jostling things around a bit gave you a better connection temporarily. This would have negated the poor connection / hum until the connection finally broke, resulting in no signal to the sub. Sean