cannot login to admin page

I’ve installed a wordpress blog on my own site, it has been working very well until recently…i suddenly find myself unable to log into the admin page! whenever i type in the username and password, the page only bounces back to the login page again. i’ve tried re-sending passwords for several times, i’m sure it’s not the problem with wrong passwords, nor is my web server down, but i cannot login to update my blog or change any settings! please tell my what’s the problem, and how can i fix it.