Moving desktop PCs whilst they are running is very unwise as your nephew found.
You don`t give any indication which cable `caught` or how or if it broke, pulled out or was damaged. If the `cable` was damaged replace it if it is necessary for operation of the PC. If a USB cable disconnect it (unless it is for keyboard/mouse). If it was a USB cable the socket may be damaged.
If you have all unnecessary peripherals disconnected and the PC still wont boot, I should check all connections to the drive. Power off at the mains before starting.
You could also try booting into BIOS (hold the <Delete> key on switching on) to see if the drive is recognised there.
There is also the possibility the OS has become corrupt, due to jarring/interruption of the drive whilst it was writing, and a `Repair` will be necessary using the Windows boot CD.