My guess would be photo-oxidation, aka sunburn. Too much sun produces the same free 0- radicals you find in hydrogen peroxide (the same ones that bleach your hair). It destroys organic molecules (chlorophyll) and turns the leaves silvery before they dry up. Sound right?

Al

yes, that's exactly what happened. They were only out for a few hours. Are they done for?

I guess it depends on what you call "done for". Tomato seedlings are almost 100% dynamic mass & carry on fairly efficient photosynthesis in the stem tissues, too. Those tissues may have also been damaged. The leaves will regrow from buds in leaf axils, if you want to wait. If you're zone 6, you still have time to start them from seeds if you want, or replant if you bought them in cell-packs.

I don't know what your plants look like now (how bad), but I would strip all the lower leaves off so there are only 2-3 pairs of leaves at the top of the plant, & replant them deep - so only the upper leaves are barely above the soil. The entire length of the stem will grow roots & you'll have a much stronger root system. I do this with ALL my tomatoes.

Al

Thank you so much for your help. I followed your advice and will watch them to see how they fare and also restarted some seeds just in case.