The misplace of the passwords is always a problem, especially if it is something important. And there is nothing more important when you have an online store and you can not log in to the backend to administrate your products.
There are several ways to reset your password:
If you have access to the database (using phpmyadmin, for example) you must go to the administrators table (it's easy to find, it has the same name), there you will see the users list and hashed passwords. Export the table on your hard disk (just to be safe, even you are the only administrator) and empty it (the sql for that is 'truncate table administrators'). The next time you access your admin log in (http://www.youronlinestor.com/admin/) you will be asked to input your name and your password to create a new administrator. If there are more than one admins (others than you) go to the exported file and copy the queries to insert them back into the table. They look like this:
INSERT INTO `administrators` (`id`, `user_name`, `user_password`) VALUES (1, 'admin', 'e5ae9781b99ddefda2be67727dd5cc96:b3');
Another method you can try is to edit your login.php file and insert a tep_db_query() call at the beginning of it in order to update your admin password to whatever you want to reset it to. Make sure you use tep_encrypt_password() to wrap around that text.
The line will look like this:
tep_db_query("update administrators set admin_password = '" . tep_encrypt_password('theNewPassword') . "' where user_name = 'admin'");
After saving the file and uploading it to the server just go to the login page of the admin console, it will automatically update that password for you, then you can strip that line of code (leave it as it was before the modification) and login like normal.