That one line is WordPress telling you the site crashed. It almost always comes from a single change: a plugin or theme that updated itself, or the host moving the site to a newer PHP version. The site is not deleted and the fix rarely means rebuilding anything. Below is a rebuilt example (not a real client) so you can see the broken page and the fixed one, then the safe order of steps.
Since WordPress 5.2, when the site crashes WordPress emails the site's admin address with the subject "Your Site is Experiencing a Technical Issue". That email names the plugin or theme that crashed and contains a recovery mode link. The link lets you log in to the dashboard with the broken plugin paused for you, so you can switch it off or roll it back. Look in spam too; it often lands there, or goes to an old address nobody reads.
WP_DEBUG and WP_DEBUG_LOG in wp-config.php, display off so visitors see nothing) and reload the page once. The log in wp-content/debug.log names the file and line that crashed, as in the example above.wp-content/plugins/booking-calendar to booking-calendar.off). WordPress deactivates it. Renaming the whole plugins folder works too, but it switches off everything at once, forms and caching included.| What the log says | What it means | Usual fix |
|---|---|---|
Fatal error: Uncaught Error: Call to undefined function in a plugin file | A plugin updated and now calls code another plugin or WordPress itself no longer has | Roll that plugin back one version, or update the plugin it depends on |
Fatal error ... syntax error, unexpected or ... removed in PHP 8 | The host moved the site to a newer PHP version and old code cannot run on it | Update or replace the old plugin/theme; PHP rollback only as a stopgap |
Allowed memory size of ... bytes exhausted | A page needs more memory than the host allows | Raise WP_MEMORY_LIMIT if the plan allows it; find the heavy plugin |
Error inside wp-content/themes/... | The theme, or a code snippet pasted into it, crashed | Undo the snippet, or switch to a default theme while it is repaired |
| Error during an update, site stuck in maintenance | An update stopped halfway | Remove the .maintenance file, re-run the update cleanly |
If you are comfortable with a file manager, the steps above are usually enough. If the site earns you money every hour it is down, that is what the box below is for.