Your site is on WordPress...
The person who built the site should provide you with a UN/PW to login to your WP dashboard admin area. If they are credible they should have no objection to providing you with the Admin UN/PW so that you can login.
It appears there is a security measure in place to hide the default login URL so they will have to advise what the new login URL is.
Your site was built with the GoDaddy GoCentral Web Builder. You can access it through your GoDaddy account dashboard. If you go to your My Products page then click on Websites, you'll find the links to access your GoCentral site dashboard there.
Then, there are bunch of helpful videos here. HTH!