Nope nope nope big glass of nope.
When you're the writer, it can be nearly impossible to distill your gigantic book baby into 250 words that will entice an agent to read on. You're too close to it, too far inside it. The urge is strong to tell instead of show, splutter redundant facts, suffer character name diarrhea, and in general ramble on long enough to make an uninvested person yawn.
And that's no good. YAWNING IS NO GOOD.
So here are the simplified bones of a great query, per the magnificent Janet Reid of QueryShark.
1. Who is your protagonist and what makes them special?
2. What is their problem? What choice are they facing?
3. Who is the antagonist and what do they want?
4. What are the stakes?
End with Thank you for your consideration and hit send.
Okay, so it's not that easy. I know how extremely not easy it is. But trust me-- whatever you have, whatever you've been querying with, it can sometimes help to open a new, blank document and answer those questions. Keep distilling until you've got it down to less than 250 words. Then ask someone who's never read the book to read it and tell you if it makes any sense.
Ask them if they care what happens.
For all the info you've ever wanted on querying, read the entirety of Query Shark. For links to every site I used to get a book deal, see the Resources tab up above. It ain't easy, but it's free.
Jump out of the nest! Press send! And may the Force be with you!