If a customer is having trouble logging into their account, the first thing to do is to check whether that customer has an account - customers who checked out as a guest will not be able to log into their account as it will not have been created.
To determine if the customer is a guest go into your store admin, and then to the customers screen (Customers menu > Customer Info > Customer List). Look at the column called "No Account". Guest customers have an X by their names.

If the customer isn't a guest customer there are two options for resetting their password:
- They can do it themselves by going to the login screen of your store and choosing "Forgot Password". It will send a new password to their email address on file.
- You can do it for them, by clicking on the customer on the Customers screen (above) and choosing "Change Password" (and following the prompts).