(we simply stayed in the car for about 3 hours everyday and nothing else,
This probably explains why she felt imprisoned. Seriously, this is all you two ever did? How long were you two a couple? (You didn't say)
To "get her back" you might want to try suggesting the two of you do something a bit more fun than sitting in a car. You know, dinner, a movie, a walk in the park or on the beach, a concert. Gee, anything other than sitting in a car!
So next time you two talk, tell her you've been thinking about her a lot also and that you'd really like to see her. Ask her if she'd like to grab a bite to eat (NOT drive thru. lol) Take it slow. Talk with her. You'll get a sense of whether she wants to try a relationship with you again. And if so, for pete's sake, take the girl somewhere.
Seriously, just sitting in your car? Really?