User has forgotten the password. They can change the password if the 'Forgotten password' option is made available to them. Alternatively, as admin you can provide the user with a new password.
Someone has changed the user's password. Try changing the password and sharing the new password with the user.
The user account may be locked. An administrator can unlock it by clicking the Admin tab >> User and access rights. Then find the user who cannot log in, click Edit, and check the Activate user account box.