I just downloaded Raspbian Jessie Lite (2016-02-26) from official website, have written the image on the micro SD card, and tried it on my headless RaspberryPi 2, with only an ethernet cable.
It doesn't boot:
the Ethernet LEDs are off
the green power LED and the red LED stay constantly on.
the power unit is working (when I use my other SD card with another install, it works perfect, and has always worked perfectly in the past since years).
As the computer is headless (I have no HDMI screen to test otherwise, and no USB keyboard, I always used succesfully SSH in the past during years), it's difficult to debug.
What could be the cause?