I apologize and redact that previous statement although it might have some merit in the end. I would recommend forcing the computer off by pressing on the power button for a couple of seconds (about 10). When it is off, turn it back on, but this time as it is turning on reset PRAM (resets the disk to which it is supposed to boot up to).
To do it, check out this article: http://support.apple.com/kb/ht1379
I would have had user use Recovery Mode, repair the system volume and also by clicking on top level of the hard drive to check partitions; then use Safe Mode on first boot.
After clearing nvram, you will need to reset the startup disk control panel.
Startup Manager: How to select a startup volume
Troubleshooting: My computer won't turn on
If your Macintosh computer won't turn on, here's how to troubleshoot it. Please keep in mind that although your computer may display similar symptoms that prevent it from showing video, this article discusses what to do if the computer won't turn on, meaning that when you press the power button:
General purpose Mac troubleshooting guide:
Starting the computer in "safe mode":
To identify potential hardware problems:
General Mac maintenance: