Don’t panic. And don’t be depressed or offended, either. This happens for a number of reasons, and might not have anything to do with your pitch.
Assuming your pitch was good (ahem) if you don’t hear back from the person after 3-4 days, simply follow up them over email.
For the love of God, do not call an editor to follow up — unless you’ve previously met for coffee or cocktails and even then, I still would avoid calling. If you don’t hear back after one follow up, you might want to take another look at your pitch. Trust me: It can almost always be made better.
Before you approach any editor, its a good idea to check and see if they have a presence on Twitter. Many do. Follow them. Then pay close attention to what they are publishing. Have you really done your homework? Does the content of the article truly fit in? Be honest. If not, if you want to get into that publication, you’ll have to rework the angle of the article before you try again.
The worst thing you can do is try to talk an editor into taking a piece that is not right for their audience. It makes you look dumb and makes the editor mad. Mad = bad.
Important: Always be extremely nice and solicitous in all communiques.