I have run into the same issue as a result of the same operation. I replaced the LCD display and now the Zune logo flashes, but only when connected to USB power. When the unit is not on external power, it appears completely dead. As doesnâ€™t respond to any keypad inputs, so I cannot apply the reset sequence detailed in KB 927001: http://support.microsoft.com/kb/927001/
I have reopened and gone back through the device. I have pretty well ruled out any errors in re-assembly, including cable hookups.
It seems like I have either toasted the mainboard or the hard drive. It would be nice if I could isolate which, since by flipping a coin, being wrong, and ending up having to purchase both of them will essentially total the original cost of the unit.
Any ideas on how to determine whether the hard drive or the board has gone bad without fancy bench equipment or cracking open another Zune to swap parts?