I just faced an issue with PHPBB forum. All forums links stopped working and was returning error 404 not found.

Not Found

The requested URL /forum-name-here-f346.html was not found on this server.

Additionally, a 404 Not Found error was encountered while trying to use an ErrorDocument to handle the request.

I remembered from history that when all links at PHPBB stop working, it is an issue with .htaccess file
And it was correct. I had phpbb_seo folder in my phpbb forum directory and in it is file "phpbb_seo_install.php"

so i fixed it by going to http://myforum.com/phpbb_seo/phpbb_seo_install.php and installing phpbb seo module.
After done, i went into PHPBB admin, click PHPBB_SEO tab, in my case: "ACP_CAT_PHPBB_SEO" and then clicked "ACP_HTACCESS" link, grabbed the code provided and pasted it into my .htaccess file which is in my forum root folder (same folder like index.php is)

Do you have similar issue / another solution?