My melons are dying - help!

Don't know what's killing them but they all seem to be keeling over and wilting. I started them in a propagator in early March, potted them on and planted in the greenhouse ten days ago. They were fine for a week and then keeled over. No frosts here and well watered, have I drowned them?

Might be worth digging one up to see if there any grubs in the soil also inspect the stem at around soil level could be damping off disease, it will look thin and transparent with no rigidity. If it is water any living seedlings with cheshunt compound. and water all your new transplants with it.