The real time Measure data will be stored on the Dynatrace AppMon server for 11 Mins during the database connection down, if your PWH issue was more than that then there is a data loss for real time and there is no impact of the historical data which is stored already in database, if you re-establish a connection to the PWH and if it's successful then the real time data will be started storing in PWH.
Step 1: Click the test connection to make sure the DB is reachable from the AppMon server.
if you are receiving any error other than test successful, then verify the server logs..
Step 2: If the test is successful then click on Connect which establishes the PWH connection.
As from the snapshot I could see the AppMon server is trying to reconnect, you can disconnect the database which enables you the test connection and can establish the connection again..
after clicking on disconnect it will enable the test connection.
Click on the test conenction
once the test is successful it will enable the Connect button.
Click on Connect and it will establish the PWH connection
You need to engage the Oracle DBA admin, as this issue is on the database.
go through the above link which will help you to know more.