You might need to invalidate a database connection and then to reconnect to it again in such cases as connection to the server being lost, etc. To invalidate a database connection and then reconnect to the database, in the Database Navigator or Projects view, click the database connection and then click the Invalidate/Reconnect button in the toolbar or Database -> Invalidate/Reconnect on the main menu:

You can also right-click the connection and click Invalidate/Reconnect on the context menu.