Assuming that this is a post iOS 5 update, the device will either reset to its previous version, come up in recovery mode (iTunes logo), or will not show anything at all.
If the iPhone is having problems correcting the issue and connecting it to iTunes doesn't help....turn off the phone, disconnect it from the computer, press and hold the home key (the physical button on the bottom of the phone) and while holding the button down plug the phone back into your computer. This will prompt the phone to enter recovery mode and should allow you to restore the phone back to factory defaults (updated OS included).
For pre iOS 5 updates the system would have to remain plugged into a computer for the whole update so power loss shouldn't really be an issue.
In the event that power IS lost the only major difference is that iOS 5 has the cloud from which you can restore a backup of your phone data where as pre iOS 5 devices will have to start from scratch.