Are you sure the flash is on? Does it have fresh batteries? Have you cleaned the electrical contacts (a rubber eraser works great for this)? Are you sure the speedlight is fully seated and locked down?
What speedlight are you using?
Nikon recommends turning off both camera and speedlight, mounting the speedlight, turning on the camera first and the speedlight second. Removal in the reverse order.
If its not communicating in full auto mode, then something is wrong with the speedlight or the camera hot shoe so a visit to the repair shop may be unavoidable. Have you tried another speedlight or tired the speedlight on another body?