My macbook pro (mid 2009) doesn't run as mac (snow leopard), only as windows 7 in the bootcamp

Hello!

I need your help please!

One day my macbook pro stops running as mac, however, it still works when I use windows 7 in the bootcamp. I would like to restore and let the macbook as the first day.

When I try to access as mac it freezes almost at the begining (with a progress bar that stop about at 7%). It is similar to this

http://bit.ly/23TMkre

Thanks in advance