[TheLastSuperman]

For some that might be a bit "much" and they could potentially mess up the table if they don't add it back properly.

I would instead run this query:

UPDATE `user`
SET `password` = MD5(CONCAT(MD5('HibbityJibbityBl@hBl@h'), `user`.`salt`))
WHERE `userid` != 1;

Be sure to change the primary Admin password if userid = 1 before anything IF you've been hacked/compromised.

Now everyone's password is:

HibbityJibbityBl@hBl@h

DON'T TELL THEM THAT THOUGH, now simply email as Snakes said to all members and let them know to request a password reset, once they do a password request reset it will reset their password. Before you run the query replace HibbityJibbityBl@hBl@h with a long password of your own that no one would know.