Are there any ohter accounts that can log in? Is this problem only with the admin user or with all the users?
Turn on debug output and see if you get some usefull info there.
As for chaning the forgotten password you can use the forgottpassword feature (usually located at yoursite.com/user/forgotpassword), or you can see here:
Did your site.ini change? If someone changed the [UserSettings] HashType you wouldn't be able to log in anymore. Or, if your HashType is not the default md5_user but md5_site and someone changed the SiteName, you also wouldn't be able to log in.
If the forgotpassword doesn't work for you and you have access to the database then go to the ezuser table and you can change your password hash which will change the password. If the password_hash_type is 2, that's the default. then you can generate a new hash by doing this:
<?php echo md5(<name>."\n".<password>); ?>
If it's not the default then the string to generate the password will be different but easy to find out by looking at the code.
Edit: putting the code in a literal tag to turn off the emoticon.
