Comfort her and be there for her. Tell her words of solace, make her something to eat, bake her something, take her for a drive or something.
ask her to explain what she is afraid of...she is 12, not 2. speak to her like you would a teenager, not a child. Panic attacks have some origin. she was not born with them. try to find out what started it.
give her a sleeping pill
Give music a try. It might calm her. If it's not working, you can also sing along. Watch a fun movie together in bed. Don't leave her alone tonight simply because without mom there, everything's different and that will just exacerbate the problem.
good question. try to know the origin. observe her, give optimistic views, tame her brave, encourage her even if she is wrong. gradually she will improve. probably she experienced worst in her childhood.
ask her what she has done in the past... redirect.
Looks like you're in for an all-nighter
you're in trouble