Hard lump-like knot on back of neck or spine. what could this be?

Lipoma: A lipoma is a slow-growing, fatty lump that's most often situated between your skin and the underlying muscle layer. Often a lipoma is easy to identify because it moves readily with slight finger pressure. It's doughy to touch and usually not tender. A lipoma isn't cancer and is usually harmless. If it does bother you the only to take out surgically.

Neck anatomy: Other than lipoma, this could also be just normal anatomy -- at the base of the neck, the C7 and t1 spinous processes are much longer than the spinous processes above them, which results in a "bump" at the cervicothoracic junction which is actually normal. Nothing needs to be done about that.