Gabbie Flowers and the Key to the Universe by Dianne Caplin

Eleven-year-old Gabbie Flowers never imagined winning the school mural contest would turn into a nightmare. She knew her friend could be mean, but she never though she had the power to make he favorite things - like flying through the night sky with her invisible friend, Samuel, and visiting her mother in Heaven - come to a crashing halt.
Samuel holds the key to making the bullying stop, but due to Gabbie's fear, she can no longer see him. Desperate, Samuel must find a way to get her attentio...