Are your plants in the ground or in containers?
What you're doing is not, strictly speaking, winter-sowing, since we put them out with no heat or protection beyond the covering over the container.
I'm sure your vegies will be fine, my arugala and lettuce have been out under the last 2 days of snow and temps in the low '30's.
Not sure about the marigolds.

The plants are in containers above ground. They are flats - seed starting trays without the lids. So the only protection is the "covering" of the cold frame. I think of it as a giant "lid" over all the flats. The heating coil does not provide heat for the flats, it is buried a couple of inches down. If the plants were buried in the soil of the cold frame they would get the benefit of the heating coil, but as they are above ground they really don't. I'm concerned that they will freeze solid. Temps will be highs in the 20s and lows in the teens for the next 5 days.

Last year I did tomatoes and marigolds successfully in the same cold frame, but it did not get as cold as it is forcast to get this week after they sprouted. So far the tomatoes have not sprouted, just the marigold and the cool weather veggies.

oh I see. :)

I would say that no worry about what has not sprouted.
If you are worried about the marigolds, toss a sheet over them, altho I think being in a cold frame, even an unheated cold frame, I think they would be fine.

Thanks ellenrr, I was looking for an experienced WSr to say "I think they would be fine". Not like I'm counting on your opinion that they will be fine, but that what I'm doing seems reasonable, and I don't have to bail and put all the flats in the garage.

One of the things I love about gardening is experimenting. Sometimes things work out the way you hope, sometimes they don't.

If all the marigolds die, I have time and seed to re-start them. Now, if all my precious tomatoes had sprouted and died - that would be a different story ;-).