This is nearly always a plugin or theme that's just updated (or an old one that doesn't like a newer PHP version). The site isn't lost; it's tripped over one file.
What to do first: check the email address registered to the site — since WordPress 5.2 it sends a "technical issue" message naming the plugin at fault, with a recovery-mode link that lets you log in and disable it.
If there's no email or the link's expired: in cPanel → File Manager, open public_html/wp-content/plugins and rename the suspect plugin's folder (add -off to the name). Reload the site. If it comes back, that was the culprit; if not, rename the whole plugins folder to disable everything, then rename it back and re-enable plugins one at a time to find the offender.
Or just tell us. Open a ticket with the domain and we'll read the error log, disable the plugin that's throwing it, or restore last night's backup — whichever gets you back fastest. Please don't keep hitting refresh or retrying the update; it won't change anything and can compound the problem.
Afterwards: update the culprit (or replace it if it's abandoned), and keep WordPress, themes and plugins updated regularly rather than in one big jump every few months — small updates rarely break anything.
