Many companies assume that their "doers" - i.e., their go-to workers who regularly meet or exceed company goals - desire a higher-ranking position. Thus, they often promote them to jobs that may offer more pay but also different and increased responsibilities. For employees who were content and effective in their previous jobs - but perhaps afraid to turn down a promotion - a well-intended reward can make them as unhappy as they are unproductive in their new positions.
To navigate what could be a touchy situation, meet with your boss as soon as possible. Express your sincere gratitude for the chance to be promoted,
and thank her for recognizing your contributions and accomplishments.
Then, explain that while you deeply appreciate the opportunity for advancement, you find your current status so professionally and personally fulfilling - and have so much you still wish to accomplish - that you must respectfully decline. Be sure to add that you hope the door for a promotion might still be open to you some day, if it becomes available again and you both feel you're ready to move on.
Don't worry that you've stalled or damaged your standing, either. Your honesty helps you avoid one of the worst fates that can befall people in their careers: being promoted before they're actually ready to move up.