I know I usually post my Weekly Chapter Challenge today, and in a way this is related, but I wanted to share this with you so you could learn from my mistake.
When I first wrote my Query Letter, it was all about the conflict and problem solving my main character had to do. Unfortunately, none of this occurred until about three-quarters of the way through my MS. So, I worked on my query until it reflected the story I had written. I ran it by several people at the Query Critique on Writer's Digest. Tweaked it a bit more, then began my search for an agent. It wasn't until I received a comment back about revising and querying that I began to worry. Enter Trisha. She looked over my first three chapter, and query letter. After some IM'ing back and forth, I came to the conclusion I should have reached after writing my first draft of my query. My story didn't begin where I had started it, that was all back story.
So, the lesson is, if you are re-writing your query because it doesn't fit the story your wrote, event though that was the story you meant to tell, check and see of you have too much back story. And don't be afraid to cut it and start somewhere else. I did, and this version is turning out to be even better-- some new characters have appeared, and my quest is more realistic than in my original version.