Help please - very sudden and odd behaviour in hen

Hi, I'm wondering if anyone can give me any advice.  Florence, one of my hens (hybrid pullet approx 26 weeks old) has suddenly - as in the last ten minutes - started displaying very odd behaviour.
She looks like she's settling down to sleep on the floor (she was under the pear tree when I found her so it's not even like she's getting ready for bed in the coop) but is totally disinterested in anything going on around her (even food/treats) and when you pick her up she just falls fowards onto her chest - almost as if her legs can't hold her weight. Her eyes are just about shut (and remain so if you pick her up) and her rump is up but her tail down.  Her legs and general temperature also feel cold in comparision to what she's normally like.
It's very odd and I'm concerned that I have a very poorly bird on my hands.  This has come from no where as earlier this evening she was wandering around the garden with the rest of the flock.
Any help / suggestions would be very welcome.
Thanks, FL

Yes, fairly regularily for the past few weeks. We have had a few softies in the last few days but we've not been able to attribute these to any particular girl.

It wasn't a soft or stuck egg - O/H went to the coop to get her for a bath but she'd already passed away.  I have no idea what could have caused a very healthy young bird to go from full of life to dead within the space of half an hour.  All I can think is she's eaten something poisonous but I have absolutely no idea what... I am totally gutted as secretly Florence was my favourite (although I love them all).  It's been an awful end to a truely horrible day.   

Oh no, poor you, what a shock - not even any time to adjust - it's horrid when it happens out of the blue like that 

Im sorry  One of my girls started showing the same symptoms as yours yesterday, I lost her on the way to the vets this morning, So sad (((hugs)))

You could be right CQ, but no doubt someone will come up with some very technical answer!

I lost one of my older girls last week.  She had been fine, but maybe not laying, she was over 3 years old, but she seemed fine one day then gone the next.

Very strange.  they hide things very well!

I'm sorry about your loss FL!   

Thank you everyone who took the time to reply to my message with either suggestions and help or words of sympathy.  It was really appreciated and has made me realise that sometimes these things do happen and it's not anything I did wrong or could have prevented.