How do i ask this girl out now?

If that's the only reason you've changed your mind then maybe you are shallow. Or maybe her new hairstyle highlights her in a different way. If you like her then ask her out. You don't have to say it's the hair that did it (although a compliment in this direction wouldn't be wrong). You could just say that you weren't previously in a position to accept a date and had some stuff to sort out, but now you are so if she's still interested, dinner would be great! Good luck.