Normally if left turns are not allowed at any instance at an intersection, there is a blatant and obvious indication of it; such as a sign or a red light. If you are facing a traffic light like the one pictured below, which had a left turning arrow that went yellow or disappeared, but still has a green light, then treat as you would any other left turn on a green traffic light. See this post for detailed information on turning left at traffic lights. It doesn’t tell you not to turn left, therefore, you can turn left as you usually would.

LeftTurn