Probably dried out. The small peat pellet should be all the way into the ground.

And most of us remove the pellet's filmy covering before we plant.

tomato is one of the few things.. where you can strip the leaves.. and plant the whole stem ...

i would have dug a one foot hole.. put the plant in the bottom.. and backfilled to show only a 6 inch plant ...

if you planted it this way due to clay .. there has to be a better way ... and you need to learn that part.. its obvious.. if you got the seed to a plant.. you worked awful hard learning that part.. but now you need to learn about planting in mother earth ...

and those peat things are God's gift to failure.. NEVER use them again... IMHO ....

also.. reflected heat off your black matting could have cooked them.. if they were not properly hardened off to sunlight ...

ken

When peat dries out, it becomes very resistant to wetting again. Even when you water it, it can run right off. I agree with both of the previous posts. Peat plugs and pellets should not be above ground level. The plant is way too tall and spindly for it's foliage. Started under inadequate light. Even with a stocky start, it's quite OK to bury most of the stem under the soil, because it'll root up it's entire buried stem. Taller, gangly tomatoes don't necessarily need to be put in one foot deep holes, you can make a trench and lay them horizontally in it and cover the stem only pulling up the top two or three inches of foliage above the soil level.

If the stems started to die near the peat level, and those pellets were too wet most likely it was from Damping Off, a common fungal disease in too wet soils.