If you have changed your database password in ServerPilot, you will also need to change it in Magento's code for your app to run correctly.
First, open your Magento app's local.xml file, located at
apps/APPNAME/public/app/etc/local.xml
Then, scroll down until you see the line
<host><![CDATA[localhost]]></host>
Here you'll see where to edit your database credentials.