To put your store in 'Maintenance' mode go to Settings>Store Settings>DIsplay>Store Closed.

You can also configure a custom store closed page to display for any customs whom get to your website while this mode is enabled.

You can also re-enable by clicking 'Open Store' at the top of your admin. You can also 'View the store as a customer' or 'admin' should you wish to use this as a development mode.