It is probably getting scorched by the sun through the glass. Most greenhouses will be getting pretty hot in his weather so bring it out to somewhere cooler and give the leaves a misting with water.

Or it could be damping off - if the seedlings started dying at one side of the container and the trouble spread across from there try watering the tray with cheshunt compound.

Hi there,

I made the mistake of pricking my lovely brasica seedlings out to grow on, and left them in the green house.....the result....
dried leaves....
All scorched.

I've salvaged what I can and resown some more, I've also draped some agricultural fleece across sunny side of greenhouse wall, like a tent, opened the vents and the door during the day.

Happy gardening.