You might want to see if the account was disabled in the admin panel. If not you the admin can change the password and send it to the users, with a direct link to the password change.
Sometimes password reset emails get caught in company filters.
You can also change their password manually by going to their profile in the admin panel and selecting "Change Password".
Next you would fill in a new password for them. Unfortunately, you will then know their new password, so it's always a good idea to prompt them to change their password upon logging in by selecting Change Password in the drop down menu under their avatar. That only requires entering their old password (the one you assigned them) and entering a new password.
Hope this helps!