• Press Customize. Cards you removed are in the Add a card tray. Click one to add it back.
• Cards for apps that are switched off aren't shown. Add the app again to get them back where you left them.
• A card that shows fewer items than you expect is probably small. While customizing, pick L for the full width or pull its bottom-right corner to make it taller, and it shows more.
• To start over, press Customize → Reset to default.

More in Customizing your dashboard.