To find your login and API key you can visit this page while logged in to bitly: http://bitly.com/a/your_api_key
To reset your API key, sign into your bitly account and go to 'settings' located in the top right corner drop down menu. Head to the 'advanced' tab and scroll down. Click on 'show legacy API key', and press the 'reset' button.
You can now use this new API key.