Here is a nice cheatsheet in the forums here that has some info on the diamond go down a bit you will find HARD RESET. it should work. If it doesn't tell us what happens if you need more help.
http://forum.ppcgeeks.com/showthread.php?t=36062
another way to reset the phone is below. but if you have no rom on the phone and the usb doesn't come back on then yikes.
http://forum.ppcgeeks.com/showthread.php?t=51508
!!! please use thanks button for anyone that may have been helpful !!!