The Geon from Villna says two things stop a person's prayer from being accepted. One if he has sins.
The other is not relevant right now.

 But if you want your prayer to be answered and you know you have done some sin , it makes a lot of sense to go to someone who in your best judgment is righteous. The problem is deciphering who is a tzadik.
being a tzadik after the time of the Baal Shem Tov became big business.