Don't worry about the XBOX not powering up, that is because the nand write did not complete.
Download JTAG Tool, click advanced, go to main tab, select LPT, select Xenon, go to nand tab, click write Xellous.
Once Xellous has been written confirm it works by booting the XBOX from the DVD Eject button, it should boot into Xellous. Once you confirmed Xellous is working, power down the XBOX. Take the new nandflash2.bin and put it onto a USB flash drive and rename it to updflash.bin then plug the USB drive into your XBOX, boot from DVD Eject, and let Xellous flash. It will tell you when its done. Once done, power down the XBOX, wait 30 seconds, then boot it normally.
NOTE: If you get E71 or E79 after writing Xellous repeat the process but instead select Write Xell.