Loop Back Test - Sticky?

MY UNO WAS WORKING GREAT .. BUT
SUDDENLY IT IS NOT RECOGNIZED BY MY PC...
I M HAVING TROUBLE WITH MY UNO R3. PC NOT DETECTS IT , I.E "NO DEVICE FOUND PROBLEM"
I CHANGED MY CABLE , TRIED ON OTHER PC BUT NO COMPORT , OR NO "NEW DEVICE FOUND"
WHAT IS THE PROBLEM ?

I HAVE DONE "Loop-Back Test"
! BUT GOD KNOWS WHY NO DEVICE DETECTED IN PC ....... HELP ME GUYS'

ADVANCE THANKS !