Plugin Locks Admin Out of WordPress

I installed the plugin and activated it.
Made my settings including activating it and saved settings.
Opened a new browser session and hit the page to see if plugin works — I got Site Undergoing Maintenance (perfect !)
Went back to initial browser and hit the page to see if plugin works — I got the site’s home page (perfect !)
Logged out of WordPress as Admin.
Hit the site again — I got Site Undergoing Maintenance (perfect !)
Tried to log back into WordPress as Admin and I cannot. Keeps going back to login page.

Closed browser and re-opened browser – still can’t login.

Now what? Help !

I then FTP’d to server and deleted plugin — still can’t login.

Not a good plugin if you ask me…