Command-w closes a window, command-h "hides" the app (all open windows and palettes etc belonging to the app will be removed from view, but they are still open - it is useful for removing all the visual clutter belonging to one app in one go from your workspace without actually closing any of your files). To bring all the windows of a hidden app back into view again, you just click the application's icon in the Dock.
For Mail, you want to display your Viewer Window again - bring Mail back into the foreground so that it is in the menubar at the top of the screen by clicking its Dock icon, and then select File>New Viewer Window.
Btw, in Mac parlance, minimising a window (or dialogue box) is the process of making it move to the right hand side of your Dock by clicking the yellow (minus) traffic light button. Maximising it is the reverse process.