A very big pine tree has fallen by the green. The grounds crew put ropes around it and it is to be played as “abnormal ground condition”. My approach shot flew into the center of the fallen tree. There is no way for me to find my ball. How do I proceed? Do I get a free drop?

Les' Answer: 

Per Rule 25-1.c: If it is known or virtually certain that a ball that has not been found is in an abnormal ground condition, the player may take relief under this Rule. If he elects to do so, the spot where the ball last crossed the outermost limits of the abnormal ground condition must be determined and, for the purpose of applying this Rule, the ball is deemed to lie at this spot... The nearest point of relief is determined and a new ball may be dropped within one club length, no closer to the hole without penalty.