I thought this would be helpful to others who use the "Maintenance Mode" plug-in while upgrading WordPress.
I first activated the maintenance mode splash page so others attending my site would see the "Maintenance" screen. Then I uploaded the files for the WordPress 2.5.1 upgrade. All files were successfully uploaded and set.
Then I went to my website on-line and saw the Maintenance Mode screen. I clicked the link "Admin Login", but nothing happened, the maintenance screen remained. I tried a link I set in my Favourites for the usual WordPress Admin Login and got a page full of code.
Seeing that I could not get into WordPress to deactivate the Maintenance Mode screen, I decided to go into files and folders of my website (via my FTP software) and changed the name of the maintenance mode PHP files to a different name like: maintenance-mode-OLD.PHP.
This bypassed the maintenance screen and I got my website back on-line.
I had to reinstall the maintenance mode plug-in files with a new set and deleted the renamed ones.
I clicked the standard Admin login link on my website and got into the dashboard of WordPress. I had to reactivate the maintenance mode plug-in the Plug-in-screen of WordPress, then go to the Maintenance screen to make sure the maintenance plugin was turned off.
I thought I would post this for others who may have had the same problem or issue upgrading to version 2.5.1 while using the Maintenance Mode Plug-in.