BeagleBonewhite Power via Expansion header

I was trying to power the beaglebone white with the expansion header - connecting 1&2 in P9 to GND and 5 and 6 in P9 to 5V. All was fine till I connected the USB cable to the Beaglebone and it will no longer work. It died instantly and without any smoke/pop sound. What could be the reason. Is it a bad idea to power it via expansion header?

That should have worked. The only thing I can think of is that there may be a grounding issue where the ground for the PC is different than the ground of the power supply you are using for the 5V input on pins 5 and 6. Make sure they are the same. Without looking at your schematic of how it is hooked up, it is really hard to say what happened.