There is a balance to be struck here. Last year, one of my assigned applicants contacted the admissions office to request a new interviewer when I did not respond to his e-mail within 3 hours. Generally, I think you need to give it a week. People do go away on business or vacation, and it is entirely possible that someone is not contactable for a week.
Beyond a week though, it is harder to justify. Once a week passes, I would mail them again, and give them a few days the second time to respond. I have to prowl my spam folder carefully during interview season, as occasionally an interview request gets misclassified as spam. If they do not respond to the second mail, then I would write to interview@mit.edu to request a new EC and tell them that their previous EC is non-communicative.
Before you do that though, check that you did not make a mistake. Two years ago, an applicant in my region wrote to his EC and did not get a response. That is because he had misspelled the EC’s e-mail address. When he did not get a reply, he did a reply-all to his originally misspelled e-mail to write to him again. Not surprisingly, he still did not get a reply. We eventually sorted it out, but he went through a lot of stress for nothing.
Weird stuff does happen though. Two weeks ago, I had a request come though that one of my EC’s was non-responsive, and the applicant was reassigned. The EC involved was the vice chair for my region. This is a man I e-mail back and forth with at least a dozen times a week. I know how involved he is in the interviewing process, and he is the least likely EC in my region to be non-responsive. Nonetheless, the applicant, for whatever reason, never got a response. I do not know how or why it happened, but I reassigned the applicant.
Of course one important observation here is that if at all possible, request an interview as soon as you are ready. DO NOT WAIT UNTIL THE LAST MINUTE to request an interview. Yes the interview deadline is 10 December, but if something goes wrong (misspelled e-mails, non-responsive EC), you may lose a week or two. Certainly in my region, if an applicant has a non-responsive EC, we will be certain to offer an interview, but if they requested the interview on the 10th, then the resulting interview will be somewhere around Christmas Eve, and nobody wants that.