Just been asked to look at a third-parties site that was working until an upgrade killed the wordpress site stone dead with the seemingly dreaded white screen of death. XBox may have the red rings and windows may have the blue screen, but when a seemingly straightforward update to wordpress goes wrong you are presented (usually) with nothing, as was the case with the site I've just fixed, nothing, no error messages, no website, no admin access...ahh!!...or not as it turned out.
By simply enabling the wordpress debug statement in the wp-config.php file I was able to trace the initial error to a missing file, taxonomy.php, which for some reason had been missed in upload or deleted by the upgrade process. After replacing this file, another error message appeared, regarding a bracket where a ; or , was expected. Not to worry, check the file mentioned and it was corrupted, a theme widget plugin file in the library directory of the theme was missing a large chunk of PHP. After re-upload of this file, bingo, the site comes back to life.
What all this goes to show is that no upgrade is simply straightforward and if you can try the upgrade on a separate (duplicate) copy of your site prior to upgrading the live site you may spare yourself some of the more spectacular failures.