No longer recognising internal hard drive, pls help

My hubby was carrying out a diagnostics check on his macbook. Unfortunately our 2 yr old got her hands on it, obviously pressed a few buttons at the wrong time and shut the screen. Hubby cannot restart his machine. He gets the big beep and then the screen is just white. He's tried to reinstall things but it is not recognising that it has an internal hard drive. Its as if she's deleted it. Please help, any thoughts?

Have you tried to booting from the install media and using the disk utility application, to see if the application sees the internal hard disk?

Yes we've done that and it doesn't see a hard drive at all.