Why can't I login to my site's admin page???

Hello,

When i go to my site’s login page to get to my admin page, which looks like this: mysite.com/wp-admin (it goes to a redirect page) I log in with my password and username, but then I click login and, instead of going to my admin page, it goes to a page that looks the same, but the url changes from the redirect page to: mysite.com/wp-login.php. Why on earth would this be happening???

If somebody could give me some help, this would be FANTASTIC!!! I can’t afford any problems because this is an ecommerce site and it is imperative that I can login to my admin page!!

Thank you