Flash doesn't change according to focal lenght
SB-600 doesn’t change focal lenght with zoom, always show at 14mm
With the SB600 and above, there is a wide flash adaptor that spreads the beam to its maximum angle of 14degrees. It rests horizontally on the top of the light. You may have pulled it out inadvertently.
Use your fingertips to see if the wide flash adaptor is down in front of the bulb. If so, it flips up from the bottom. Lift it back up and tuck it into its pocket on top of the bulb area.
When this screen is back in position, you will have full mobility again.
If however, you have broken off this screen, you may be able to retrieve mobility with the insertion of a small piece of card (business card?) into the slot where the wide angle adaptor used to be. You will probably have to tape this in place. If this is not the problem and the adaptor is in its storage position, you will have to take the flash to a skilled technician to have it serviced.