PHB p. 291. Second Wind can only be used once per encounter. Normally, this takes an action, so you can't do it while unconscious. The DC 10 Heal check allows another character to trigger your second wind without costing you an action, meaning that it allows you to use your second wind while unconscious. You are, however, still using your second wind. Ergo, if you have already used your second wind and are knocked out later in the fight, an ally cannot use Heal to trigger your second wind because you've already used your second wind.
No, I said that second wind was a healing effect. If your second wind is triggered and you have no healing surges left, you are healed to 1 hp.
If your second wind has already been used, however, you cannot use it again whether you have healing surges or not.
To use an analogy, imagine that instead of triggering your second wind with a Heal check, the cleric is using Healing Word on you. If the cleric still has a use of Healing Word this encounter, you get healed--either to 1 hp if you have no healing surges left (per the rule on p. 295) or to your healing surge value (plus 1d6 from Healing Word and the cleric's Wis bonus from Healer's Lore, but that's not really relevant). If, however, the cleric has already used up all of his Healing Words for this encounter, you cannot receive healing from it.
It's the same with second wind. Not having the healing surge to spend as part of the source of healing is not the same thing as not having the ability to use that source of healing in the first place. Although it's not given a formal power write-up, Second Wind is effectively an encounter power. Once you've used it, you can't use it again until you take a short rest.
