I think you have in mind the attack on the Pieta in St. Peter’s basilica. Apparently it was repaired so that the damage is not visible (but now tourists can’t get close to the statue). I never heard that the statue of David was attacked.
Yep both by Michalangelo and two of the greatest ever. The idiot who attacked the Pieta with a hammer no less was a crazed Aussie Hungarian which I also happen to be. Never had an inkling to ever do any vandalism in my life though.