If witness dies do I need a new will?

Sorry for the loss of your friend.  Typically, the death of a witness will only become an issue if someone decides to contest your will after you die.  In that case, the judge will want testimony or sworn statements from the witnesses about what they saw and heard when you executed your will.  If one or both witnesses are dead, the court will not have access to that testimony.  However, in Massachusetts, most witnesses to wills sign a "self-proving affidavit."  Here is an example:
 
We, the undersigned witnesses, each declare in the presence of the testator that the testator signed (or directed another to sign for him and such person signed for him) and executed this instrument as his Will in the presence of each of us, that he signed it willingly (or willingly directed another to sign it for him), that each of us signs this Will as a witness in the presence of the testator, and to the best of our knowledge the testator is eighteen (18) years of age or over, of sound mind, and under no constraint or undue influence.
 
The self-proving affidavit is admissible in court in the event of a will contest.  So the short answer is that you do not need to execute a new will or revise your current will just because a witness dies.  If, for some reason, your will does not contain a self-proving affidavit, you might want to consider executing the will again, especially if you have litigious heirs.