“Fatal error: Uncaught Error”

“Fatal error” means PHP hit code it couldn't run and stopped building the page. PHP is the language WP runs on; your host sets its version.

• A plugin calls a function that isn't there, often after an update, or when a plugin it needs is switched off.
• 2 plugins, or a plugin and your theme, use the same function name. The error says “Cannot redeclare”.
• Your theme's functions.php has a mistake, often from code pasted in from a tutorial.
• Your host moved to a newer PHP version that an old plugin or theme can't run on.
• A page used more memory than PHP allows: “Allowed memory size … exhausted”.
• A page took longer than your host allows: “Maximum execution time … exceeded”.
• A plugin update stopped halfway and left some of its files missing.

How to read the message

A full fatal error line has 3 parts. First comes what went wrong, such as “Uncaught Error: Call to undefined function”. Then the file, then the line number after a colon.

The file path is your best clue. A path with wp-content/plugins/ points to the plugin whose folder comes next. A path with wp-content/themes/ points to your theme.

A path with wp-includes or wp-admin is WP itself, and WP is rarely the cause. Look down the lines under “Stack trace” for the first plugin or theme folder.

Where do you see it? With debugging off, visitors only see “There has been a critical error on this website.” The full line goes to your error log.

Many hosts show that log in their control panel, often under Logs. Or turn on WP's own log with the 2 lines from the steps above, then open wp-content/debug.log.

Fix it by cause

A plugin

Switch it off in Plugins › Installed Plugins: press Deactivate under it. If you can't log in, rename its folder in wp-content/plugins instead.

Then see if an update fixes it, or go back to the version that worked. The plugin's support forum on wordpress.org often has others reporting the same line.

A “Cannot redeclare” error names 2 files. Those are the 2 plugins that clash. Keep the one you need, and look for an update of the other.

Your theme

Rename your theme's folder in wp-content/themes. WP can't find it, so it switches to a default theme, such as Twenty Twenty-Five, if one is installed.

Did the error start after an edit to functions.php? Put back your copy of that file, or undo the edit in the file manager. Then switch back in Appearance › Themes.

A new PHP version

Did the error start the day your host changed PHP? Then an old plugin or theme can't run on the new version.

Ask your host to set the old version back for now. Then update the plugin, or find one that's still kept up to date.

Memory or time

“Allowed memory size … exhausted” means a page used more memory than PHP allows. Add define( 'WP_MEMORY_LIMIT', '256M' ); to wp-config.php, above “That's all, stop editing!”. Some hosts cap it lower, so ask yours.

If it comes back at a higher limit, a plugin is using too much. Switch plugins off one at a time to find it.

“Maximum execution time … exceeded” means a page took longer than your host allows. It's often a big import or a slow outside service. Ask your host what the limit is before you raise it.

A half-finished update

If an update stopped halfway, the plugin's files don't match. Rename its folder, then install it again from Plugins. Its settings stay in your database.

Stop it coming back

Update one plugin at a time and open your site after each. If it breaks, you know which one did it.

Keep Administration Email Address in Settings › General on an inbox you read. That's where WP sends the link that lets you log in with the broken plugin paused.

Keep a copy of functions.php and wp-config.php before you edit them. A copy on your computer is enough to put them back.

When you're done, remove the 2 debug lines from wp-config.php. Otherwise the log keeps growing, and on many hosts anyone can open it at yoursite.com/wp-content/debug.log.