If it's part of the main stem, something happened to it just below the wilted part. Follow the stem down from the tip as you look for a hole from critters and/or a break (or fracture) of the stem.

Be sure the soil is moist but not soaked. Take it out of the "greenhouse" for good and start in the house by a window that catches morning sun (continuing supplemental lighting the whole time). I'm thinking your "greenhouse" is some sort of plastic dome, which should have been removed shortly upon gemination. When it holds up to that for several hours indoors, then start it gradually outside, and always begin use morning sun when it is more humid and lower in the sky. Pictures would help as mentioned but I think the answer will still be the same. Hopefully the plant is healthy and has had sufficient light to develop so far. A fan or breezy place is also helpful if you have one by a window. Good luck :) PC

A familiar echo from Dave above.. Transplanted plants often "pout" a little until they settle in lol.. Be sure to water them well immediately after transplanting, and it doesn't hurt to give them a few days of shade to stretch their legs a bit. Container soil must drain readily.. You'd be better off purchasing something like Promix, Sunshine mix, or making a mixture of your own. A simple, cheap and effective mix would consist of something like compost, triple ground hardwood bark and pea gravel/silica at equal parts. You'd want to add a dry fertilizer (GardenTone for example) at the time of mixing if not using a soluble product throughout the season.

People need pictures to offer help.