From time to time your users will not be able to login to their account, this is almost always a user error, rather than a system issue, there are 3 ways that a user wont be able to log in: 

1) they have locked themselves out by entering the wrong password 10 or more times

- this can be checked in the admin > tools > users > select user > system tab

you can change the value to 0 and save if they have more than 10 in this box

2) they are trying to login as a 'full' account, but have only previously used the quick checkout feature (one-page-checkout only)

- if they have only used the quick checkout before then the 'live' checkbox will not be ticked in the

admin > tools > users > select user > system tab

You shouldn't tick the live box for these users though, they don't have passwords, and as such they aren't meant to be able to log in


3) they are trying to use the quick checkout, but have a 'full' account (one-page-checkout only)

Users cant use the quick checkout if they have a full account (a full account is one that is live with a password)


Users can reset their password using the 'forgot password' link


* assigning an order to a user while using the system as an admin user (eg in the one-page-checkout) has no effect on the user account, and doesn't need a password