hiya, sorry your pumpkin seeds didnt take...
ive no idea why really and this sounds so obvious lol...but maybe they got too hot? I know in a greenhouse they need ventilation ...

my pumpkin plants which are now taking over my conservatory were planted a couple months back.... on window sill...but i didnt trnasfer to biger pots until they were quite big....long tendrils...etc...not sure if this was right thing to do but it worked for mine

i did notice some wilting when it began to get hot and i hadnt thought to ventilate the conservatory....when i did they picked up great and have been going great guns since.....just plenty of water and lots of fresh air lol...

ive recently sowed some more...bout a week ago...already theyve got two leaves each....and il prob do the same again...pot on when much bigger...mind you these arnt for me..ive no room for them but i just seem to able to produce big heathly pumpkin plants lol....well so far lol.

Maybe you could plant some more...i put mine in 3.5" pots and then a propegator lid on top..water well and left them to it lol...couldnt beleive how fast theyve grown!
Good luck x

Another possibility, Ceregh, is cold damage! from what I've been reading, there has been actual frost in the last few weeks there in the UK--even us desert dwellers are impressed by the temperature swings! Anyway, if the damage was done a couple weeks ago, then the greenhouse may have been too cold for them, if unheated and already ventilated. Of course, if this just happened in the last few days, it's more likely to be what GGM said.