Hello!
I was using my arduino UNO (official) r2 for a robotics project. After a night of hard work, i continued my work in the morning but the arduino would not get detected. I had used the same usb cable last night.
I would really appreciate the help as this is my 3rd arduino and being a student in a poor country (india) this is the last arduino i can afford to break 
Here are the symptoms:
1) The Arduino is completely NOT detected. I have tried this on 2 laptops, both mac and win7. When the arduino is plugged in, the laptop does not detect ANYTHING!
EDIT: On the last night that this UNO was working i randomly got this message a couple of times : avrdude: stk500_getsync(): not in sync
but the issue usually resolved it self by plugging/replugging/reset button.
2) I have tried it with 2 USB cables. The USB cable which was working before has also stopped working
3) None of the uC's i.e 'MEGAU8 and 'MEGA328P get warm
4) Since the arduino is not detected at all, the loopback test cannot be performed
5) The previous sketch runs fine
6) When i connect the arduino to power (usb or external) the on board LED 13 blinks (i think it blinks 3 times though not sure)
Help is much appreciated as i have many competitons coming up :/