Do you have a picture you could post? Without one it's tough to say but I'll try

I see 2 possibilities...the 400 watt light might have been too close and they weren't able to drink as fast as they needed because the root structure wasn't recovered fully since it was cut...or maybe the nute strength was way to high for such tiny plants...I'm not sure how much food you can give tiny plants but a lot of people say not to give any food till you see 4 nodes you could probably get away with a tiny amount of food but when they are small you have to be careful in the future I would not put a 2 inch tall plant next to a much larger plant if they feed from the same res...

It's possible they aren't dead yet if you can flush them out with just water a few times they may recover alright I had the same problem and mine flopped over like they were touched by the reeper only to perk back up again once I fixed the nutes

If ya can't get a pic offer up some more info on light distance, nute strength and what the setup is like were they in the same res with much bigger (hungrier) plants

best of luck