and you have gotten into the bootloader before, right?
go into clockwork and flash the bootloader a few times just to make sure. use bootstrap after that and see if it works.
if not,
try the recovery failsafe. (always has worked for me anyways.)
1. remove the bettery from your phone.
2. turn it on and let it boot as normal. While it is on the boot image (the "cartoon" that shows after the moto logo, but before the lockscreen) pull the battery. leave it out for 3-4 seconds, and then put it back in and power up. this ALWAYS gets me to the bootloader if i ever have a problem