I've been a teacher for a very long time. Here are a few thoughts:
1. give the teacher a bit of grace - chances are high that they didn't receive your first email (happens more than you think) because the email got stuck in spam/junk.
2. the teacher may be overwhelmed with post holiday email
3. it's possible the teacher did see your email and didn't realize you wanted a response
4. yes, it is possible the teacher is just not responding and yes that is super annoying
5. Do NOT email the HOS. This a ridiculous reason to contact the HOS and they will think you are overreacting.
6. You might consider emailing the department chair to check in
7. you might also consider a phone call in which you verify that the teacher received your email.
You would not believe the number of times a parent sent me a scathing email asking why I hadn't responded to an email that either went to spam (for some unknown reason) or that they actually found later in their own draft folder and hadn't actually sent.