We all know the first line of a query is your "hook." I call the last line the "sinker." You want it to punch them in the face, in a nice, friendly kind of way that makes them unable to forget you after having read the 300 other queries in their inbox.
If you're looking for query advice, but are slightly intimidated by my claws, blade, or just my rolling googly-eyes, check out the query critique boards over at AgentQueryConnect. This is where I got my start, with advice from people smarter than me. Don't be afraid to ask for help with the most critical first step of your writing journey - the query. My comments appear in green.
Niya Coleman killed herself on October 3rd, the night of homecoming, because all she wanted, more than anything in the world, was nothing. Good hook. The only thing I'd suggest would be perhaps eliminating the exact date to make the sentence flow a little better. The idea that it's homecoming is enough.
Unfortunately for Niya, the first thing she discovers about death is that it’s the opposite of nothingness. It’s full of all the sounds and lights and people she hated most while living and, consider breaking up this sentence right here. It's very long. even worse, all the things people hated most about her — like her bluntness and her aversion to touching and the fact that she has to rely on her handy emotion dictionary to remember that lips turn upwards when people are happy, downwards when they are sad, and purse when angry— have all followed her to the grave. And now, just as in life, Niya finds herself in a special class, confined with hundreds of other spirits to the Suicide Room.
The Suicide Room, with its blindingly white walls and endless rows of waiting room chairs, isn’t for all the (d?)early departed. It’s only for those poor souls who had the misfortune of dying before fulfilling a purpose. A purpose so important that the universe would be even more damned if it goes unfulfilled. A bit wordy again. Consider blending these two sentences concerning purpose and ing a little more economy of words.
Guided only by Hawtha, the ancient and secretive host of the Suicide Room, and Bernardo, the always smiling boy who needs help testing his theory that memories, a hidden door, and a dangerous visit to the living world lead can lead to fulfilling purposes, Niya has to choose between the lesser of two evils. She can focus her energies on discovering how she’s meant to help her rapist and the sister she spent her life wishing away, or she can risk not fulfilling her purpose, never getting her nothingness and spending eternity in the Suicide Room.
The Suicide Room is complete at 85,000 words. It is a YA, fantasy Hmm.... not sure this qualifies as a fantasy. Look into the genre of magical realism and see if that might be a better fit novel centered around a girl who has broken from the weight of her differences, and now has to examine her past to rebuild a life she’ll never get to live.
Overall this is quite good. I did some strikethroughs to illustrate spots where you can practice more economy of words, but that's my biggest nit. Look for spots where you can tighten up - even slightly - and you're in good shape. The concept is sound and interesting!