Follow these steps (original guide found here)

#1. Clear your cookies
Cookies are in your computer.
In windows they are in
C: / docs and setting / username / Cookies folder.

#2. Clear the temp cache on your computer

#3.Edit wp-login.php
Go to the “case ‘retrievepassword’:” section.

Just under the comment (In 1.2mingus, the line in question is number 204):

// redefining user_login ensures we return the right case in the email

Replace:

$user_login = $user_data[’user_login’];

with:

$user_login = $user_data->user_login;

#4. Your blog host should provide you with phpMyadmin access to your databases for you to do this.
Open phpMyadmin, and click on the users table in the left menu (it could be wp_users, or yourprefix_users)
Click “Browse”
Click on the edit button next to the admin user.
Delete whatever is in the password field.
Now go to the md5 hasher
enter your desired password, and copy the hash that it generates
Paste the hash in the password field for admin
Save the info in the database.
Login to Wordpress using “admin” and the password you used to create the hash.
A guide to doing this is at Podz’ site

#5. Secure HTTPS ?
Change the following line in wp-includes/vars.php from:
define(’COOKIEPATH’, preg_replace(’|http://[^/]+|i’, ‘’, get_settings(’home’) . ‘/’ ) );

To:
define(’COOKIEPATH’, preg_replace(’|https?://[^/]+|i’, ‘’, get_settings(’home’) . ‘/’ ) );

Be sure also to change the url in Options to https://
(Credit for this solution to mcnicold)

#6. Head over to Sexygirlgeek’s wp-login fix
Copy the all the text from
Save it as ‘wp-login.php’ (Credit for this solution to sexygirlgeek)
Upload it to your wordpress directory
(If you get an error about ‘headers already sent’ after doing this, go here: Solving the headers already sent problem.