If login issues then Reset your Account Password
How to reset your Account Password?
1. On your storefront, click the Sign In or Login link. Most BigCommerce themes have this link at the top of the page, and some themes will display it in the footer as well.
Example of this link located in the footer.
2. To the right of Sign In, click the Forgot your password? link.
3. Enter the email address associated with your customer account, then click Reset Password. If a valid email address was submitted, you will see a "Thank you" message.
4. Within five minutes you should receive an email with the subject line, Password change request for [Store Name]. Click the link in that email.
5. Enter your new password twice, once in the New Password field, and a second time in the Confirm Password field, then click Continue. You should see "Your password has been successfully updated."