It could be the tube or an internal issue. Remove power from the lights and wait 10 miniutes to let them cool and discharge any stored energy. Carefully remove the dome and flash tube from the non working light (the tube just pulls straight out, but gently rocking it may help).
You will likely see dark spots inside the glass near the metal legs. This is fine. However, any breaks in the glass or the legs would cause the tube to fail.
If you see any breakage, you can use a replacement tube or use the tube in the other light, in the non working light. If the light now flashes, then it was simply a tube issue. If the light still does not work, then there is an internal problem that will need to be fixed under warranty. Please visit this page for information on returning items for repair or service:
http://www.paulcbuff.com/repair.php