Does your query have:
1. Dear Janet (or equivalent salutation)
NOT: Dear Agent, To Whom it may concern
2. The title of your work
3. The kind of work: fiction, non-fiction, memoir.
4. The word count
(under 50,000 is too few, over 200,000 is too many)
5. Two paragraphs showing me what the book is about and enticing me to read more.
6. The first 3-5 pages of the novel or memoir pasted below the query letter
(do NOT send an attachment in a query letter. I will not open it)
7. Your contact information: email, phone number
These are the basics.
If you don't have these, I can't evaluate your query and thus it will be rejected.