Repair WordPress Homepage 404 Error

I have just put a brand-new WordPress site online, and the moment anyone types the main URL they hit a 404 page. Internal links and the dashboard load, but the home page itself will not resolve. I haven’t installed any recent plugins or theme updates—this is a fresh build—so the issue is likely buried in permalink settings, .htaccess rules, or an overlooked server-side redirect.

I need you to:
• Diagnose the exact cause of the home-page 404 that appears when visiting the root URL.
• Apply the fix and confirm the page loads normally across desktop and mobile.
• Make sure your solution keeps search-engine-friendly URLs intact so rankings aren’t hurt.
• Give me a brief note explaining what you changed so the problem doesn’t return.

You’ll have full wp-admin and hosting-panel access the moment we start, and I’m hoping to see the site live before the end of the day.