Like any web platform, application, or operating system, WordPress can be hit by sudden unexpected problems. Fortunately, general troubleshooting is easy and will usually reveal the source of (and sometimes fix) most problems. Before we continue to general troubleshooting, there are a few popular cases that have relatively specific fixes.
If you’re seeing a blank white screen, consult WordPress and the White Screen of Death. If you’re seeing “Fatal error: Allowed memory size,” consult WordPress and the Fatal Memory Error. If you’re seeing an internal server error or error 500, consult WordPress and the Internal Server Error. And, if you’re seeing a headers already sent warning, consult WordPress and the Headers Already Sent Warning.
As you can probably guess from the articles linked to above, many sudden WordPress problems can be traced back to plugins, themes, or corrupt core files. If your problem is not directly related to one of these cases, it’s time to move through the general troubleshooting steps to find the cause of the problem and maybe even fix it along the way.
If you have access to your admin panel, try deactivating all plugins. If you don’t have access to your admin panel, try manually resetting your plugins. If that resolves the issue, reactivate each one individually until you find the cause.
If your blog showed no signs of improvement after deactivating your plugins, then it’s time to move on to your currently active theme. If you have access to your admin panel, try switching to the Default theme (WordPress 1.5 – 2.9.2) or the Twenty Ten theme (WordPress 3.0 and higher). If you don’t have access to your admin panel, access your server via FTP or SFTP, navigate to /wp-content/themes/ and rename the directory of your currently active theme. This will force the Default theme (WordPress 1.5 – 2.9.2) or the Twenty Ten theme (WordPress 3.0 and higher) to activate and hopefully rule-out a theme-specific issue.
If your blog showed no signs of improvement after deactivating your plugins and switching your theme, try downloading WordPress again and delete then replace your copies of everything except the wp-config.php file and the /wp-content/ directory with fresh copies from the download. This will effectively replace all of your core files without damaging your content and settings. Some uploaders tend to be unreliable when overwriting files, so don’t forget to delete the original files before replacing them.
If the general troubleshooting steps have failed to identify the cause of the problem, the volunteers in the WordPress Support Forums will be more than happy to help you, especially if you list all the steps you have taken so far.