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 will email all your users 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)
1) Download the emergency-password-reset plugin
2) Upload it to your plugins directory and activate.
3) If you are and administrator you can click on “Reset all Passwords” – the plugin will generate new more secure passwords and let all the users know.
I advise you to change the SALTS first in wp-config.php using wordpress.org SALT generator – that forces everyone logged in off!