How to reconcile a failed prophecy.
First I understand prophecy to be the foretelling of a future event as revealed solely by God to a human being. By definition that means that when someone says "God told me this or that about a future event" it means that it will happen at the appointed time with 100% accuracy or the prophecy is false; not the Bible or God, but the prophecy itself or the one that spoke it are false.

(KJV)Deuteronomy 18:21-22
And if thou say in thine heart, How shall we know the word which the Lord hath not spoken? [22] When a prophet speaketh in the name of the Lord , if the thing follow not, nor come to pass, that is the thing which the Lord hath not spoken, but the prophet hath spoken it presumptuously: thou shalt not be afraid of him.

An example of a true prophecy would be when Joseph warned the Egyptian King Pharoah that there would be a famine in the land for several years and to stockpile all their goods in preparation for the famine. This led Joseph ( an Israelite) to be the 2nd highest in command over all of pagan nation of Egypt.

In our day more and more we are plagued with the question WHY. Why did mommy pass away, or how did little Jeffery get this disease or why can't I have this or that for my life. In truth we're desperate for answers from the future sense in order to make our present situations enjoyable by avoiding the pitfalls or traps of the unknown. In desperation humanity is willing to accept any person that has the boldness to say “I have seen the future and I am willing to declare it”. There are several problems with that, Man is fallible, meaning man makes mistakes.

Our society is trained to accept EVERYTHING that the best of the best tell us. For example if Michael Jackson gives you dance lessons or Lebron James wants to help you with your jump shot we will listen intently and follow every instruction to the letter or we'd be a fool not to.

But what about the greatest speakers for God the studied theologians...can't I trust them... the soft answer is yes...but the Bible explains how. First of all the Bible teaches us Let God be true and every man a liar... in other words no matter what man teaches us we must rely on God's Holy spirit to reveal it to us as truth. That truth can be revealed to us in various forms... the Bible tells us in times past God spoke to us through angels, prophets etc, but in the Last Days he speaks to us through His son JESUS the absolute symbol and authority of absolute truth. What that means to me is if Jesus hasn't revealed it to my heart, that warm fuzzy feeling of peace, than I'm not going to put my trust in it.

Then how can I know truth and trust?

(KJV)Proverbs 3:5-6
[5] Trust in the Lord with all thine heart; and lean not unto thine own understanding. [6] In all thy ways acknowledge him, and he shall direct thy paths.

We can trust the Lord, if you are diligently seeking after the truth from God, you have nothing to worry about.

What went wrong on May 21, 2011 was unfortunate because people put their trust in a man. Nowhere does the Bible explicitly say On May 21, 2011 an event will take place. No matter how many verses and mathematical equations and bible codes man can create Jesus trumps them all by saying NO ONE KNOWS THE DAY OR THE HOUR.

I encourage everyone to read God's word the Bible possibly start with the Gospel of John it tells the story of Jesus (much of the movie Passion of the Christ, was taken from the book of John)