It’s possible that someone (something) deleted all of the entries in the wp-posts table. If you are comfortable working with phpMyAdmin, you could take a peek there.
It’s also possible that your current theme might be ‘borked’ for some reason. So the advice on changing to one of the ‘twenty’ themes is a good idea. And a plugin might cause problems with accessing the wp-post data.
I’d try first changing to a ‘twenty’ theme. Then disabling all plugins (especially those that you have added lately).
If you are still having problems, then it’s possible that your site was hacked. I’d look at the generated page source to see if there is anything that shouldn’t be there.
I’d also change/reset all access to the site: WP users, FTP users, Hosting account users, etc. A fresh install of WP, along with a fresh install of your current theme, might also be a good idea.