basic information like.... WHAT HELI IS IT? would help others give you some advise.

even without that you obviously don't have rudder control. check and make sure the gyro is working, the rudder servo hasn't been moved, the rudder servo is still plugged into the gyro and gyro plugged into the receiver, make sure the belt tension is correct and it's not slipping on a gear (at the tail or at the front), make sure the rudder servo hasn't had it's gears stripped ...etc.