How old is she? Two of my aging hens have taken to sleeping on the floor tucked under a nest box, snuggling together and looking pretty darned cozy. I leave them alone since they aren't hurting anything.
Most of the time they don't poop, but sometimes they deposit some during the night, easily scooped up in the morning. It's not a health problem as I see it.
There are times, though, when a hen will stopped trying to roost due to conflicts on the perch at roosting time. If they're low in the pecking order, sometimes they just give up and quit trying to compete for a spot on the perch and they sleep where they can find a peaceful spot.
You might observe your chickens at roosting time to see if this might be the reason your Wyandotte isn't roosting. I've hung divider curtains at strategic intervals along the perch that I can unhook from the ceiling and let hang down between two hens that want to fight. It really helps keep the peace.
