I just remember this tips today. Last year, when I was upgrading my iPAQ hx4700 (I’m stil using it right now) to Windows Mobile 2005, somehow the system was stucked. I didn’t know why, but it couldn’t boot up. Just showing boot up screen but never showed the Today Screen.

I remember that iPAQ (my old one, the 36xx) has a unique Parrot Mode (or debug mode in common terms). I looked around the web to find the shortcut button to enter the debug mode. Finally, I found the shortcut. The keys are “AddressBook + iTask + Reset” button. All pressed on the same time. Your screen will show HP logo with no backlight. Connect it to your PC (USB/serial) then re-flash your hx4700 either with WM2003 (original ROM) or with WM2005. Just run the ROM executable to re-flash your hx4700.

Another example: XDA
I also have another experience with my XDA IIs (HTC Blue Angel). Well, I played with some ROMs from XDA Developer and few times my device locked up. And I have to re-flash it back. The only way to solve this problem is by entering the debug mode by pressing the “Soft Reset + Power + Side Navigator” button. Then, connect your XDA to the PC and re-flash it back.

This method will work to fix the “dead” XDA/iPAQ due to corrupted ROM and power failure during flashing process.