DS went through a little bout of these a while ago.
Frightened the flippin life out of me at first, as he was "awake", then I found out about night terrors and figured it wasn't anything to worry about.
Make sure they can't hurt themselves (eg, if they get out of bed are they gonna fall down stairs, or whatever). Don't actively try to comfort them as they usually get more distressed if touched.
Don't try to wake them.
Sit in the doorway/close by and talk to them in a low, calm voice.
Doesn't matter what about, just mumble at them.
After a while they should either actually wake up, in which case you can then put them back to bed, or, they'll just turn over and go back to resting sleep.
The difficulty for me was the inability to comfort him, but once I realised this was a night-terror thing, not a failing-as-a-mother thing, it didn't worry me any more. It's horrible to see them through it, but remember it's worse for you than it is for them. They know nothing about it. Xx