D3 is a 5.6V zener diode. Its purpose is over-voltage protection. Here is what I suggest: desolder D3, and then measure the *resistance* between +5V (VIN) and GND. If the resistance is somewhere in the range between 4.4K to 5K, that means the switching regulator is still likely to work. If not (for example, only a few ohms), the switching regulator has been damaged, and I suggest that you send an email to [email protected] so we can arrange for a replacement.