Peter Nix has already quoted the rules on this. Perhaps it's easier to explain with an example combat.
Situation #1
Initiative sequence:
- Monster
- Cleric (already sustaining consecrated ground) - outside the zone
- other party members
If the monster drops the cleric to negative hitpoints on its turn, the sustained power will end when the cleric makes his death save (which is the only thing he can do on his round afaik) because the condition demanded by the rules (the action spent to sustain the power) hasn't happened. No matter what the other party members do will allow the power to be sustained.
Situation #2
Initiative sequence:
- Monster
- other party members
- Cleric (already sustaining consecrated ground) - outside the zone
If the monster drops the cleric to negative hitpoints in this situation it depends on the actions of the other party members whether the sustained power will end or not. If e.g. the wizard successfully administers a healing potion to the cleric or the figher/mc warlord uses Inspiring word or something like that, the cleric is conscious when his turn comes up and able to meet the sustain condition by expending the required action.
Situation #3
Initiative sequence: either from #1 or #2, but this time the cleric is inside the zone
In this situation the healing from the zone kicks in when the unconscious cleric starts his/her turn. Therefore the cleric is conscious by the end of his turn and was likely able to spend the required action to keep the sustained power running.
The only thing I can think of right now that would allow the Cleric to keep his power running was a Sustaining Cloak (DDI-Link) which could work because using its encounter power is a "no action" action. At least that's the way I read it. Item's source: Published in PH Races: Tiefling, page(s) 29
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