I've collected it all again so that it all stays together - if I have missed anything, let me know and I will add it here.
Another summary of what's around the forums.
Try to login after each step.
#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
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
$user_login = $user_data['user_login'];
$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 on the edit button next to the admin user.
Delete whatever is in the password field.
Now go to http://bfl.rctek.com/tools/?tool=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 here: http://www.mfr.f2s.com/phpmyadmin/
#5. Secure HTTPS ?
Change the following line in wp-includes/vars.php from:
define('COOKIEPATH', preg_replace('|http://[^/]+|i', '', get_settings('home') . '/' ) );
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 http://www.sexygirlgeek.com/share/wp-login.phps
Copy the all the text from <?php to ?>
Save it as 'wp-login.php'
Upload it to your wordpress directory
(If you get an error about 'headers already sent' after doing this, go here: http://faq.wordpress.net/view.php?p=7
(Credit for this solution to sexygirlgeek)
If these steps fail, please say this when posting and give details of your server setup if you know it, mysql and php versions, OS and browser, WordPress version.