If you (or the website administrator) recently updated the DNS settings of the website or registered a new domain, please note that it can take up to 48 hours for these changes to take effect, so, the website may appear that it's down during this time.
Note: If 48 hours have passed and nothing changed (i.e. your website is still down), contact the hosting-server operators.
Maybe the website is down because of an ISP- or hosting-related issue. Try again in 10-30 minutes to check if it's back online. If it's still offline, report the issue to the hosting provider which will resolve it quickly.
Check and modify the .htaccess file(s)
If you have already verified that all the file permissions are correct and you still see the 403 Forbidden error, you will need to check your .htaccess file. You can have multiple .htaccess files in your account, so you may need to search or modify .htaccess files other than public_html/.htaccess.
For example, if you have an addon domain located at public_html/addon_domain, you'll need to check any .htaccess files within that folder as well. If one of the lines in the actual file says Options All -Indexes then simply remove -Indexes and save the file.
Note: This solution also helps if the "504 Gateway Timeout" error and "500 Internal Server Error" are displayed.