Most probably you have a plugin installed that is blocking access to the wp-admin page and redirecting it to your front-end login page. There may be a way to bypass it depending on the plugin.
One thing you can try to do is just logging in using your regular admin password on the front-end customer login page. The plugin may be using the built in WordPress users functionality to implement front end user accounts and it would automatically allow you to access wp-admin once you’re logged into an admin account.
If that doesn’t work – try finding out what plugin you’re using to block the login access (what plugin are you using to implement the front end logins?) – then look up a way to bypass it.