Go to Plugins > Add New.
Under Search, type WP Secure Maintenance
Find WP Secure Maintenance and click Install Now to install it
If successful, click Activate Plugin to activate it and you are ready to go.
2.Wp Secure Maintenance Setting
You must first set up the settings:
You Can Change the Logo
Change Logo Height & Width
Label For Submit button
Place Holder Text
3.Lock Your Site Front End
After Set Up The Settings Your Site Will be lock And display like as below.