Arduino Zero DEBUG and L LEDs stuck on


I tested the example "Blink" program when I my Arduino Zero arrived in the mail, and it worked as expected. However, when I run the exact same Blink program now, both the DEBUG and L lights are turned on. It also seems that calling Serial.println is causing DEBUG to turn off momentarily, before switching on again.

When measured, the voltage across pin 13 and GND is approximately 2.8 V, so I assume that pin 13 is working fine. I also found that connecting pin 13 and GND turns the L light off. That said, I had previously connected a data ground wire (for a Neopixel) to pin 13 by accident, but none of the lights were actually on.

Please let me know what might be the problem--thanks!