Stumbling chicken. Stands but can’t walk

I have an older hen (5+) who upon returning from a trip is lame. She can stand but can’t walk, she stumbles backwards. She is eating and drinking fine in isolation. She had laid a few eggs in the last couple of days. Her eyes and everything seems to be good. I have her on a B vitamin supplement and additional protein but she doesn’t seem to be improving. I’m not asking for guesses but if someone has experienced this I’d love to hear the outcome. Cheers all!