macbook pro not booting after installing parallel windows 7

i installed a parallel windows 7 in my macbook pro lion x for sometime both of them worked properly but i had a submission so used windows continuously for a week and then when tried to start mac os it didnt boot the loading sign appeared but the login window did not. i even tried keeping it that way thinking it might show the login window later but it didnt now i am not sure how to boot the laptop with mac os though windows is working fine