Attach an external hard drive and reboot the system with Command-R held down to load the recovery drive. This will ensure a factory-default software environment in which you can try applying the firmware update.
After the firmware update is installed, you can open the Startup Disk system preferences and select your main boot drive again.
In addition to checking the power, try resetting the Mac's System Management Controller (SMC), as this controller unit is responsible for how power is handled in the system.
These files will restore your Mac to working condition; however, before you can use these files, you must copy them to a CD or DVD.Apple supplies firmware updates from time to time, and although very few people have any trouble after installing them, problems do crop up now and then.The most common problems are the result of a power failure during the installation process, or turning your Mac off during the installation because you think it's stuck.If not, remove any residual traces of past update attempts by opening the System Firmware Updates directory and moving any items in this folder to the desktop.Then try downloading the relevant updater manually from the Apple Support Downloads page and run it.Then, if something goes wrong during a firmware update, you can restart your Mac from the Firmware Restoration CD and your Mac will replace the corrupt firmware with the known good version.