Uno R3 suddenly not being recognised by Windows

I have the latest version of the Arduino IDE. My Arduino Uno R3 board (on COM 3) was working fine for many days.But now the board is not recognised by the computer. COM 3 does not show up in Device Manager when Uno is connected to PC.
The on-board LED as well as the ON led is on.
PLease help.

Same thing just happened to my Leonardo board... I'll be watching for responses