Find out why the child is scared. Was it a bad dream? Did s/he see something in the window? Did s/he hear something?
Then just snuggle your child. My boys are 2 and they have bad dreams (well I think that's what it is, they wake up crying) so after calming them down I just hold them.
Children occasionally have nightmares. When that happened I would put the child in bed with me and we would both go to sleep. It didn't happen often, but it happened. The occasional nightmare is not abnormal.