Our dog runs off for 2 hours at a time when we let her out. We tie her up when she comes home to punish her but this is not working. What can we do?

We have a dog that is a mixed breed and just a wonderful dog. She loves people, but yet will bark if someone pulls into our driveway. She's not aggressive at all and like I said just a very loving animal. She is now almost two years old and in the last week or so, when we let her outside she disappears for two or three hours, sometimes longer. She used to come right back when I would whistle for her but not lately. When she does come home, she's not panting or wet like she has been running. How can we break her of this? She hates to be tied up and we've done that when we caught her sneaking home. It's like an embarassment to her. Is there anything else we can do?

Thank You,

Laurie Stewart