This approach relies on a) comfort with the command and b) CLI access to your web server.
ssh into your web host.
(If you don't have SSH, get a new web host. If this is beyond your competence, then it's feasible to modify your local copy of the relevant file, and upload it, and then repeat that to undo the change - but it's a bit more faffing around.)
Rename the index.php file in the application's root directory - to, say, index.foo.
This alone will take you off the air. For added points, you can pop in a temporary file - either index.php or index.html (both should work equally well), with contents like this (adjust to taste)
<html> <head> <title> Site off air temporarily </title> </head> <body> The site is off the air temporarily - please try again in an hour. </body> </html>