WordPress Website Error Repair

My WordPress sites has suddenly gone down and visitors are met with a glaring error message instead of the homepage. I need someone who can jump in right away, trace the root cause, and restore the site so every page loads normally again.

Approx 15 sites
- Show a critical error
- or cannot login to the sites

Here’s what I expect:
• Pinpoint the error that’s blocking the site—whether it’s a corrupted plugin, theme conflict, PHP issue, or database hiccup.
• Fix the problem, test thoroughly, and confirm the site is fully accessible on both desktop and mobile.
• Provide a brief summary of what went wrong and any preventive steps I should take moving forward.