Hen can't use roost - where should she sleep

Try making a nice cozy nest on the floor for her. It may take a while for her to get used to it. Block off the nest box at night only and pick her up and put her on the new nest.

 

If you place her on the roost, does she stay? Is it a balance issue, or getting up to the roost issue? Does she fly? Assuming she hops about on the ground - you could make a ramp for her up to the roost. Another thing to consider is what are your roosts made of? If they are small / narrow, she may not be able to grip them with only 1 foot. My roosts are 2x3's with the 3" side up, so it is wide enough that they don't have to grip it, and can sit on their feet to keep them warm in winter. I have a hen who tries to sleep outside in the run every night, so when I close up the coop for the night, I pick her up and bring her inside and place her on the roost next to the others. I think she enjoys being held, so she waits for me to come get her!

 

Depending on her balance, creating a lower roost that has a ramp to it might be good. I'd say block off the nesting boxes but depending on if you can get back out there early morning to open them, that wouldn't be a good idea. I usually have 1-2 eggs in the boxes before I even open the coop. You could put the nesting boxes higher so she couldn't get in them, and then create a sleeping/nesting box for her on the floor.