Contract extensions must executed prior to the date that defines them.
Extending a closing date is a fairly common practice, issues do arise on many deals. You did not mention the reason for the closing date passing, but you should try and work out a reasonable solution with the current buyer.
Starting over with another buyer is not going to guarantee you any smoother of a deal, so why not deal with a bird in hand?
If you want to elaborate on the reason, we may be able to give you a better resolution for your issue. But if you desire to cancel the deal, wording a cancellation addendum can be found online I am sure, or pay a local Realtor to draw one up for you. But be warned there may be repercussions with you canceling the transaction with the buyer and the agent.