I’ve optimized my Database with a plugin named Advance Database Cleaner (which you shall avoid at any cost). It deleted the site database tree, which resulted in an immediate crash of the site.
In order to restore my site, I setup a clean installation of WordPress and then installed this plugin. However, it won’t activate. Perhaps their servers are down or something.
As a result, I cannot restore my site!
I use the newest version of WordPress (5.3), which is compatible with this plugin.
Regardless, this is quite much a poor practice – to be dependent on a live WordPress site, in order to restore a WordPress site which just crashed… I.e. to restore a backup, I must first setup another instance of WP.