Contacting university after graduating for help contacting a professor who isn't replying to emails
you should not contact the head of the department or any other Prof. just to get in touch with a professor, that is just a bad idea on many different levels.

If you are really desperate, I would perhaps send an email to one of his PhD students and ask if he has been around for if he is on vacation. As another resort, you could try calling the secretary of the department and ask if they have seen this Prof. around. Client physically to the professors office does not seem like a good idea either, unless you set up an appointment. If you know the professor well enough that I would perhaps consider just calling his/her office number directly and during office hours. some people acknowledge that they are bad at answering emails, so in that case I do not think he would mind a cold call.