Hi Tilly
You need to get this little one out of her hidey hole and set up in one secure room or even better a large dog crate. Put all the neccessary items in there: bed food water litter tray and toys.
A cuddly toy she can sleep with would be nice but make sure its kitten proof as she may suckle on it and could choke on threads or eyes.
Is she eating and going to the toilet ok?
If it were me i would go to the vet and buy some kitten replacement milk, its called cimicat or lactol and at the same time get advice on worming.
Have you any pictures of the little one? And whats her name?