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.
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.