Suppose I've readied an action to attack a minion, and then I was damaged below 0hp and fell unconscious. On the next turn I was healed. Do I still have the readied action to hit the minion or did it disappear?
You do not lost your readied action by the rules, but you lost your readied action when your next turn comes up, regardless of whether you are conscious at that time.
This means if the trigger to attack the minon happens after you are healed but before your next turn, you can still do the attack (most likely while prone, and only if you can still attack). Otherwise, the readied action is lost.
It doesn't matter if the trigger happened many times while you are unconscious. It's the first trigger after you are healed and before your next turn that counts.
(Ready an Action, Rules Compendium p.247) If the trigger doesn't occur or the creature chooses to ignore it, the creature can't use the readied action and instead takes its next turn as normal.
Since you are unconscious, you cannot percept the trigger, and I understand it as "the trigger doesn't occur (to the creature, i.e. to you)".
There is no rule in 4E that says you lose the readied action. Nothing under the Unconscious condition, except you are not aware of your surroundings, therefore could not react to the trigger if it occurred during your period of unconsciousness. I.e. if you trigger was "when the creature steps through the door", and you wake up and it has already come through the door, then you cannot react logically (as far as you are concerned it may as well of teleported to its current location)
You will most likely have fallen prone, which might interfere with or prevent your readied action. For instance if you wanted to Charge at the minion when it was within Charge range, this would no longer be possible (some feats or items might fix that specific problem)
I think it may also be a reasonable DM's judgement call to say that the readied action was lost. I personally would not argue that this was unfair, provided it was consistent, and it is very situational. It is not unbalancing to make a decision either way.