The girl I got from Whiterun....suddenly disappeared halfway through a dungeon....

Is she dead? Can I bring her back to life? She was kinda useful and I miss her

If you really want to know, go to a mercenary or someone else that can follow you. (ex: Warrior blonde woman in Whiterun tavern). If they say your party is too full, she is still alive and gyou have to wait a few ingame days. If she joins you, Lydia has died.
Good luck

This has happened to me a couple of times but she turned back up, she sometimes gets seperated, I thought a dragon had killed her in the wilderness with me one time and I knew I'd never find her body , but three or four fast travels later there she was.