HTC Desire - phone's dead
I don't know what your app did, but I find it strange that it is no longer booting. I would suggest trying to get your device in to recovery mode. I have never done this on a non-rooted device so I am not 100% sure you can do a factory reset from the "stock recovery". But I know you can do it on a rooted device.

I found these steps, but again, I am not sure if this pertains to only rooted devices.


Turn your desire off, Now hold the volume down and power button in to boot into hboot.
Now use the volume button to navigate to boot recovery and select it by pressing the trackpad button.