She could have slipped or something that you didn't see, or maybe something hurts, or a vision issue, or nothing. Things change when they get older. Lila was the easiest dog to do nails, she would flop over on her side and fall asleep when I dremmeled. Then one day she had a full on panic attack about the dremmel. Nothing had happened, I can manipulate her paws otherwise with no issue. So she could have just decided that she doesn't like those stairs anymore. I hope you're able to find something that works so she can continue to get to her bedroom.