How do I activate the window again when I am running Pinjo as a service?

If you are running Pinjo as a service you can activate it again by double-clicking the green checkmark trayicon. That icon is located somewhere next to your clock on your taskbar.

Also when you are running Pinjo as a process it minimizes to the traybar.
It could be necessary to click the arrow icon (<<) on the left side first to show all available icons.