I had that same problem, Anna. In fact, I had it twice. The first hen lived, the second died. The first was a perfectly healthy EE pullet, and one day she was found lying on the ground being pecked by the others. I saved her, though, but she could not stand. During the next few weeks she slowly regained the ability to walk, but she was very wobbly, falling over all the time. She's fine now, just a little wobbly.
The other hen was like the first (she was a campine) in that she had serious trouble balancing. She was a little better a few days later, but about a week after this, she grew very sick. She would barely eat, and had diahrea and a drooping comp and everything, and after a few weeks had a painful death.
Both were not contagious. I've no idea what the problems were. I hope your hen ends up being like the first. Is your hen sitting down, not even able to stand?