Though I haven't seen this happen to my Windows machines (I don't feel like waiting for a couple of seconds for them to turn off so I just unplug them) I would assume that suddenly removing power can damage files on any system, especially vital system files. Sometimes programs that are set to run when the machine boots can get messed up and prevent the system from booting.
You said that you can boot into safe mode (Unless I'm misunderstanding you), I'd first try to back up important files. If that works, I'd try to uninstall recently installed programs.
If that still doesn't work, you might be able to either boot to the install CD or open it from directly from the desktop and use it to repair the install. I'm not familiar with the Mac OS X install disks so I'm not sure if it's possible but it would be my assumption that it is. If it's possible, you probably do it from either directly inside the CD (When you boot the Mac into safe mode and view the disk's contents) or you can use the Disk Utility in the CD when you boot to it.
If that still doesn't work you'll unfortunately need to wipe the hard drive and completely re-install Mac OS X.
