The same thing happened to me over this weekend.
The firmware update didn't even fail, it appeared to complete. But the unit powered down during the installation (as it was supposed to, according to the installation guide), and now it won't power back on. I wonder if there is some bug with the no-press buttons?
I called Samsung and they told me that I could mail it back (shipping at my cost, $150 parts and labor estimate) to have it "looked at" since the warranty is up.... the firmware on this is obviously an issue on their end.
Ideas / responses appreciated!!