Hi, I've contact my hosting support and my site is going back to normal...
and in case anyone had the same problem like mine, here's what I did...
1. Get into your site PhpMyAdmin page
2. Klik your database name on the left, and find <prefix>_users
3. Klik 'edit' on your desired username
4. In password section, enter your new password and don't forget to give MD5 function on that password
5. Done, and you can login with your new password
It turns out that I forgot my own password... That's weird, I logged in with this usual ID and password... Damn, maybe I should take that vitamin...
Anyway, thank you before...