serial port missing from pc

i have an arduino uno R3, and my pc is a windows 8. i plugged the board and uploaded the program and it worked fine but then when i again plugged in the board, it doesnt shows the COM port. the green light on the board is on and the led blinks juast as i programmed it before but the pc doesnt reads the COM port. i also saw in the device manager but there is no COM port for the board. please help!!!

Close the IDE and replug the board. If that doesn't help: Reboot windows.