The general case of a sudden failure.
WHAT CHANGED AT YOUR END
An update applied, possibly automatically A plugin installed A file edited A setting changed A developer worked on the site
Automatic updates are the most common answer people overlook, because nobody did anything deliberately.
WHAT CHANGED ELSEWHERE
A certificate expired The domain expired A payment failed and the service suspended An external service you depend on changed or failed A PHP version was updated
CHECKING
cPanel > Metrics > Errors, for when entries began File Manager sorted by modification date, for what changed Your email, for update notices, expiry warnings and invoices
THE THREE MOST COMMON
An automatic plugin update breaking something A certificate or domain expiring Disk or inodes filling up
Each produces sudden failure with no action from you.
WHAT TO DO
Establish the time it started, then look at what happened around then.
The answer is almost always in that window.
