I just posted a
step-by-step guide for recovering from a "botched" upgrade. Most frequently this applied to users who were attempting a BIOS upgrade when the commercial power went out (and they weren't on UPS power), and subsequently they could not boot their device. The power failure turned their unit into a very expensive brick.
Most users who used this guide have indicated that it did resolve their problems. This guide is one that I created while I moderated at another website a few years ago.
Use at your own risk (although with it failing to boot now, it's not like you have much to lose). Let me know if it works for you!