May I add to this tip? Especially don't panic if the Hub's LED is green (C5/C7) or blue (C3/C4). Also, even if a C5/C7 LED is blue, this is still most likely a recoverable situation.
Good point, that reminded me, to add another one, that's rare, but does happen. Check the hub's LED and if it's red, unplug the hub for 30 seconds and then plug it back in.