Q: What should I do if my story is accepted but the editor goes silent?

In February I was notified that a story of mine was accepted for publication in an "upcoming issue" of a magazine. They told me the terms and asked for a bio. I responded that day and asked if they knew when it might appear. No response. I waited a month and politely asked again. Still no response. I realize they may have a backlog. But now, after three months with no further word, I'm concerned. This story had been submitted to numerous contests (all with reading fees) and other publications, but I withdrew it. Any ideas of what to do?