Get a fresh copy of the Joomla version you had on your site and re-upload all the "administrator" directory files. If anything there was deleted, this will possibly replace it and get it working again.
Next, check the topic I posted above for the link on how to reset the Admin password and try that.
Hopefully you have a backup. You may want to just install your backup copy of your site, files and database as hackers can inject things into your database as well so you'll want to start fresh there.
hey ben thanks!!! im a littel worried i dont know if it is relevant but this letters appear at the top of the banner of the admin page
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do you think it is some kind of attempt to hack my site???
thanks
and by the way, no backup at all!!!!!
Don't know. That is really just a closed tag though so that can happen with code screw ups. As for being sure if your username and password is right, one thing some hackers do is get in by guessing it and then change it. That's why I said to look for the link for changing the admin password.
You can always try asking your host if they have a backup for you.