I was upgrading the WordPress site for someone and had a few moments of panic when, after upgrading, all I could see were blank pages. Visions of having to go through the pain of reinstalling the database from the backup, and uploading all the files from the backup, were dancing through my head, which would turn a quick upgrade into a long marathon. The upgrade here was from 2.6.something to 3.0.1, and I hadn’t bothered doing all the intermediate upgrades, so that made the prospect even worse.
Poking around the various support pages encouraged me to try a couple of different things first. The fact that all the pages were blank, both the admin site and the publicly-visible site, made the problem seem worse than it ended up being. And the solutions turned out to be relatively simple.
Step 1: get the admin site going. I’d made all the plugins inactive, but following the advice on the WP FAQ troubleshooting page, I renamed the plugins directory to plugins.hold, and created a new empty plugins directory. This worked, and I could see the admin site. It turns out that one particular plugin created havoc even when it wasn’t activated. I could then reinstall all the needed plugins cleanly from the automatic install one at a time, testing to make sure each one worked.
Step 2: go to the Appearance page and turn on the default theme (one thing I’d forgotten to do before upgrading). It turns out that the old theme wasn’t compatible with 3.0.1, and showed only blank pages.
Now the site works again, albeit not looking quite the same as it did due to the theme, but that problem is tractable and doesn’t create anywhere near the same “oh, no” problem that the others did.