My dell 670 having problem at starting  .

This problem is happen when i was doing an experiment to extend my wifi usb cable length the Black wire of usb connector attached on front panel usb port the wire is striped and suddenly touches the external 20 VDC adapter Black wire after this the computer is instantly shuts off. 

Motherboard AUX light and Start Light is on continuously when i power up the system .

front lights , A B C D . off

Power light on as Solid Yellow.

System fans . off

Can anyone give me technical assistance Pleas.