Restore Down WordPress Site

My WordPress site has suddenly gone offline and is now showing an error message instead of the usual pages. No plugins were updated and no theme files were touched just before the crash, so I suspect a corrupted or missing file is to blame.

What I need:
• Pinpoint the exact file or configuration causing the error
• Repair or replace it so both the front-end and wp-admin load normally
• Confirm that themes and plugins still function as expected once the site is live again
• Send me a brief, clear summary of what you fixed and any preventive steps I should take

Please be comfortable working through cPanel or direct FTP, navigating PHP logs, and restoring sites quickly. Availability to start right away is essential—the site is currently down and every minute counts.