To fix this, please, CHMOD your public_html directory to 755.
You may do this in different ways:
1. Go to your Control Panel and click on Fix File Ownership
2. Enter Control Panel -> File Manager. Choose your public_html folder and change permissions by selecting "Change Permissions" button.
3. Use your FTP client (for example FileZilla) to connect and change CHMOD with it.
More detailed tutorial on how to fix 403 forbidden error can be found here.