If a user enters the incorrect password more than 3 times the system could lock their account and prevent them from logging in.  In order for them to be able to log in again you'll have to unlock their account. To do this:

1.   Go to Control Panel - Users - Manage Users
2.   Type the username into the search field 
3.   Press Enter or click Search
4.   Locate the specific user and click to highlight
   
5.   Once the user is highlighted, scroll down to User Information
6.   Click Security and choose Lock/Unlock
   
   Note:  A locked account will have a yellow padlock next to their avatar.
   
7.   If you've reset their password after unlocking their account provide the user with the newly created password, they should now be able to log into their account