msi ati 482 vga port no signals

i all,
recentlly my pc stopped to responed .
the monitor say there is no signals the pc is started and stay on,
but it doesnt do any sound and the monitor is black so i dont see anything.

i must admit that my 13 years boy took the cam usb cable and connect
it to the pc with both side of the cable(dont ask how and why) ,
but after this the pc worked ok but when ever i plugged a usb
device the pc freezed.

please help.
d.t