My dog's back legs gave out?

A few minutes ago I went to check on my 13-15 year old dachshund and saw he had fallen out of his bed. He was shaking and looked like he couldn't get up. I tried to put him back in bed but he yelped loudly when tried to pick him up, he usually doesn't do this. His legs have given out before, but my parents aren't home right now to help. What should I do?