A few moments ago I was using the RC2 version of Wordpress. The Dashboard alerted me to the new ‘Coltrane’ version which is the final release for the 2.7 cycle. In the RC2 release it gave me the option to ‘Upgrade Automatically,’ so I figured, well, might as well give it a try.
Being a new feature the first thing I did was backup the database (MySQL 5.0.51a) and within Eclipse I backed up the file tree itself to a mirror just in case. This is all on my local development machine and your environment will vary. After being alerted that the download and install had gone as expected I reloaded the back end to find that my local site was now under maintenance and I could not log in or view the front end either. The only thing that would display was the ‘Site Under Maintenance’ message.
This seemed rather odd to me but not surprising. I ran a difference check (diff) on the database along with a diff on the file tree. Once again, creating a backup before you upgrade your site within any system be it Wordpress, TYPO3, or any other content managment system makes the difference between being totally screwed or acting like a professional. From the diff I found a .maintenance file which appeared to be left over from the upgrade procedure. After deleting this from the new file tree my site was now running Wordpress 2.7 ‘Coltrane’ just fine.
So, if your site is stuck in Maintenance mode after you attempt to upgrade check your file tree for ‘.maintenance’ and delete it. If this does not return your site to working order it is probably time to consult the bug tracker at Wordpress and restore your site from backup. You did backup, right? Take care and have fun. I hope this helped.