1. If you can log in to your Zen Cart admin using another account, please do so, and reset/modify the other admin password as needed via Admin->Tools->Admin Settings

2. If you can't remember your admin account or password, there's still hope.

The first thing you can try is to click the "Resend Password" button and enter the admin email address.

If for some reason that doesn't work for you, you can create a temporary admin account in order to log in.

Open your phpMyAdmin (supplied by your hosting company), click the "SQL" tab and run this query:

DELETE FROM admin WHERE admin_name = 'Admin'; 
INSERT INTO admin (admin_name, admin_email, admin_pass, admin_level) 
VALUES ('Admin', 'admin@localhost', '351683ea4e19efe34874b501fdbf9792:9b', 1);

If you are using a prefix for your database tables, you'll naturally need to add that prefix to the table name above.
ie. "... FROM/INTO prefix_admin ...".

You should now be able to login using the following details:

Username: Admin
Password: admin

Be sure to use proper case. ie. 'Admin' for username, not 'admin'.

After you log in, remember to delete this temporary admin account and create a new one from Admin->Tools->Admin Settings