This is, unfortunately, a not too uncommon occurrence in my limited experience in top US graduate schools. Some professors are over committed;others might be having a real life emergency/crises - yet others are just not good at replying to emails. There are a number of different strategies that you could employ at this juncture.
1. Email again, early next week:
There are a few variations to this strategy. First, if the professor has just missed your email due to a veritable deluge of emails in her inbox then this could be a timely reminder. Second, if there is another collaborator, temporary graduate chair or other, initial faculty that you worked with (since you mention that you are a second year graduate student), then an email with such a person cc'ed is usually good incentive to get at least, an initial reply.
2. Meet with the department chair/other goto faculty:
Every department will usually have one faculty who serves as a point of contact for students in any kind of a predicament. This is a predicament. You need to kickstart your research agenda with this professor (potentially) but have not been able to do so. In our department, we call them "Director of Graduate Studies". Your department might have a similar or different nomenclature for such a person. An email sent from this person (cc'ing you) is usually enough to get a reply.
3. If all fails, lie in wait to surprise them IRL:
Figure out their office hours. If they teach, then they must have office hours and usually, a website for the course that they are teaching. Ask their TA's (if they have any) when the office hours are. If you see this faculty in the corridors then stop them and chat with them.
I am a fifth year graduate student and over the years, I have had to (unfortunately) employ most variations of the three broad strategies I mentioned.One reason why I chose the dissertation committee that I have is that these professors always make time for me if I want to chat with them about some academic (and sometimes non-academic) issues.
I hope you succeed in your quest to locate said faculty. Good luck!