Website down? It’s possibly been hacked. Check with your website host first to make sure the hosting itself is still running ok, but then it will be a case of checking website files, and in particular the .htaccess file for evidence of hacking or change. Download/FTP a copy of your .htaccess file and view/edit with a html editor (ie Dreamweaver), upload and recheck website.
The .htaccess file can sometimes be modified by upgrades of website or plugins, so as usual – please make backups of the website before doing any upgrades.
If your site has been hacked, make sure you change all usernames and passwords where possible, for FTP, WordPress site and cPanel hosting etc