You can put the blog in maintenance mode, which will be noted as a 503 if there is access to the url.
See https://b2evolution.net/man/maintenance-html

Alternatively and could provide a better experience to visitors.
If you have access to cpanel or can ftp to the domain/public_html/ then make a temporary [index.html] with whatever you like, a nice picture maybe, and change the endings of any existing [index.php] and [index.html] by adding and x at the end or move them to some other directory like [/temp/]

Thank You that did it!

Which did you do? :)