If your wordpress gets hacked somehow, you should reset all your user’s passwords as a precaution. there are some other things you should do here. Read about it!
This plugin does just that! It creates new passwords for every user, so hackers are stopped and then it will email all your users a password reset link, so they can create a new password – so watch out if you have more than a hundred on a shared host (they won’t like a mass email out)
It’s available on the WordPress repository here
I advise you to change the SALTS first in wp-config.php using wordpress.org SALT generator – that forces everyone logged in off!