The potential advisor suddenly stopped answering my emails. What should I do now?
At some universities, there is a quiet period between the application submission deadline and the first round of interviews, during which faculty and applicant communication is discouraged. 

As faculty, I've found such quiet periods annoying but understand the rationale which is to level the playing field by suppressing off-channel communications with a secondary purpose of limiting too much badgering on the part of applicants. 

Or the professor could be skiing or preparing for the new semester. Either way, I'd hesitate to badger too much. 

update: If I found a prospective student used an email tracking program on me, I'd drop them from consideration immediately. See: Is it rude to use tracking softwares for the emails that you send to potential advisors?