Ideally, you will have a great working relationship with your supervising instructor of record. However, sometimes communication can break down. Here's what to do when it does.
The Director of Graduate Studies (DGS) of your program: This is generally the person who determines TA assignments for your department. The DGS should be the first person contacted for most situations involving graduate students. You can find who your Director of Graduate Studies is by visiting your department page.
Department Chair: If your instructor of record happens to be the Director of Graduate Studies of your program, or you feel uncomfortable approaching them for any reason, the next person to contact is your Department Chair.
If you feel uncomfortable approaching either your Graduate Director or the Department Chair, you can reach out to the Arts & Sciences Graduate Ombudsperson. This person is appointed by the university as a neutral facilitator for conflicts involving graduate students and faculty. To the extent possible, information provided by students will be kept confidential. No action will be taken without a student's explicit consent.
The Arts & Sciences Ombudsperson is a faculty member and may have a relationship to you or your graduate program. You can instead visit the University Ombuds. Or, check out this advice from the University Ombuds on a variety of topics.