Once you have deleted a screen or a screen set, there's no recovering it, unfortunately. You'll just have to reconstruct it. This is usually not hard if it is a screen set (you just need to add back the appropriate screens, in the right order, and with the right conditions), but can be much harder for a screen.
Based on your last message, it sounds like you've deleted the screen named "Main Screen". Rebuilding it is probably the only solution.