I had this happen to one of my extensions. Here is what you do:
2. Reply to the email that was sent to you with a request for explanation (you can recycle what you wrote from the form).
3. If you do not receive a response within a week, submit another form.
4. When they do reply to you, make sure you find out the exact reason your extension was taken down.
5. The reply may tell you which file from your extension they have a problem with specifically. Try to fix this file and resubmit your extension as an update.
Do not panic. This has happened to me and I can tell you that as long as your extension is not malicious and follows their guidelines, your extension will be reposted.