Why did my guy friend e-mail me back just one word--"hey"?

Sounds like he was drunk, wanting to say more and seeing if you were out there! Just mail him and make a joke of it, ask him if he's taken a vow of silence or something!! That way you'll be reopening the lines of communication without seeming too pushy. There's a chance he might not even remember mailing you but it does tell you he was thinking of you.