Fix Missing WordPress Pages

My WordPress site is showing the homepage only—every other URL I try returns an error, with Contact Us and Services being the most urgent. I have not made any recent changes and everything running is straight out-of-the-box (no custom themes or plugins). The installation itself is fairly old, so core, theme and plugin versions are all behind.

Here’s what I need:

• Identify why the interior pages vanished (permalink break, database issue, .htaccess, or anything else).
• Restore full page visibility so every public URL loads normally.
• Bring WordPress, the active theme and bundled plugins up to their latest stable releases after confirming good backups.
• Leave me a short summary of the root cause and the steps you took, so I can reference it later.

Access to the WP admin dashboard and hosting panel (cPanel/FTP) is ready as soon as you ask. Success is when every page displays correctly, no errors appear in the logs, and I can safely run future updates without the site disappearing again.