when you say they stopped working - were they reporting as off-line AND unable to control them through the app? if only off-line, rebooting your hub through IDE usually resolves it.
if both, perhaps you can try powering off one of the bulbs for a moment, power it back up and see if it reconnects. if it does, perform the same on the remaining ones.
a couple other users have reported issues after the hub firmware update so contact ST Support