Now that you know whether you have a preface or a prologue, you have to decide if you really need it.
If you read ANY agent blogs or follow agents on Twitter, you will see that a good majority of them HATE prologues and prefaces. A lot of them even say it turns them off of the manuscript completely. Many of the agents I follow say they can't gauge your writing, your voice, or the tone of the book based on a preface or prologue. Also, think about one of the cardinal rules of writing. You never dump backstory into your first chapters. You always want to craft the backstory into the plot throughout the book. But, by using a preface or a prologue, you're essentially dumping backstory on the agent in the first few pages.
My personal opinion is that I don't mind prologues, but I despise prefaces. A prologue serves a purpose for the story. You need the information from the prologue to understand the rest of the book. But a preface is just a retelling of something (usually part of the climax) that happens later in the story. I've had so many authors tell me they included a preface because they felt like they wouldn't be able to pull a reader in with their first chapter. If you can't grab a reader with the first few pages of your first chapter, you're certainly not going to be able to get an agent's attention. I've even read that some agents completely disregard prologues/prefaces sent to them. Agents are trying to get a feel for your writing style, voice, and the plot. They can't do that from a prologue/preface. If you're worried that your first chapter, or even your first few paragraphs won't keep a reader's attention, you need to rewrite.
Now I know you're thinking that if prologues and prefaces are so unpopular with agents, why do so many books have them? I've wondered that myself and this is my theory...I think a lot of editors like what a prologue or preface can do for a book. Most people buying a book like to scan the first few pages of the book before they buy it. If there's something interesting in those first few pages, a reader is much more likely to buy the book so they can find out what happens. My day job is in marketing/advertising/PR and from a marketing perspective, the preface prologue strategy makes a lot of sense. I’m sure there are other reasons as well--like maybe to help the audience understand an important part of the book or something intense or emotional the character is going through--but like I said, from a marketing perspective, it’s a very good idea. So while agents may not like prologues and prefaces, it's smart marketing on the side of the editor and publishing house.
So where does this leave you? Well, if you already have a prologue or preface, take a serious, unbiased look at it and ask yourself if it's really important enough to your book to risk the possibility of turning an agent off by sending it with your query. If you choose not to include your preface or prologue, it doesn't mean you have to give it up. Keep it around because an editor might want it someday. :)