|I've never ever had any trouble due to WD or MSE definition updates.|
Try this crazy sounding idea - at least it's easy:
Turn off computer and remove power cord. Remove main battery too if it is a laptop. Before re-assembling hold the Power Off/On button down for at least 20 seconds to discharge the motherboard components. At odd times this procedure has unstuck computers.
Maybe it was just coincidence that it happened after the update.
Always pop back and let us know the outcome - thanks