Hmm. Can't find anything in SM2 or errata and my draft URD from 3DG just has the standard rule.
4th edition says pretty much the same as 3rd but does have a couple of extra bits including, "Armour and cargo holds cannot be lost to maintenance failure. Shields lost to maintenance failure cannot be repaired by damage control."
All a bit ambiguous. Personally I'd say that the shield generators have failed, so the shields are down. They can't be brought back up by emergency repairs or standard shield repairs.