Web browsers have recently added a "Not Secure" notice to the address bar of websites that aren't accessed over a secure HTTPS connection, as a way of reminding people that it's not safe to share personal information like passwords or bank info over a "standard" unsecure and unencrypted HTTP connection.
Nothing has changed with your website, it hasn't been hacked, etc, it's just a new feature browsers have added to encourage people to be more mindful of their online privacy.
Luckily, 22Slides offers free and automatic SSL certificates, which allow your site to be accessed over a secure HTTPS connection, protecting your site's visitors' privacy and showing a "padlock" icon in their address bar.
To make sure your website always uses HTTPS, login to https://22slides.com/signin and click your "Settings" tab. There, you'll find an "always use HTTPS" option you can enable.
Once you do that, your site will always be accessed via an HTTPS connection, the padlock shown in the address bar, and the "Not Secure" message will be nowhere to be found.