Should I be worried? Hen staying in coop

[Kirstie1978]

My silkie bantam chicken has spent most of the afternoon in the coop and is very reluctant to come out when called or for corn. I k ow it's been warm the past few days, could that be the cause? My other Orpington bantam is black and fine and spending her time on her own around the garden. Any suggestions? Should I leave her outside tonight or bring her in?? Help!!! I don't want to loose the last of my hand reared chicks
Thanks
Kirstie