I've have this problem a couple of times after upgrading Wordpress to the latest version. Trying to go to the admin page in Wordpress, I received the message that the page does not exist with the following line in the address bar, after my site name
I do not remember what I did the last time, I believe I reinstalled the update from Goddady's site. But this time around it did not work. After doing some research I found this at the Wordpress site:
The post by chowsir had the solution for me. The way I see it it is very simple, if you read the line. The page that does not exist is upgrade.php, so I look into my wp-admin directory, and it is true, the file is not there, there are other files with similar names update.php, upgrade-functions.php, update-core.php, but the file upgrade.php is not there, at least, not anymore.
Go to http://wordpress.org/download/ and download wordpress again
unzip the files and go int wp-admin folder
copy the file upgrade.php to your site.
I received a message about the need to update the database, gave it a click, and now is working again.
Hope it helps, if not, now I know what to do the third time around.