Arduino Board Connecting And Disconnecting After Connect To the PC or power Sour

When i connect My Arduino UNO Atmega 328p board to the pc or a power source, ON Led and L Led are turning on and L Led is blinking. And Computer recognise the device is connected. And All leds are turning off within 4 seconds or less and computer recognise that device has been disconnected.
I tryd connecting board to another computer and same thing happens. Does someone know What is wrong with my board?

Do you have anything connected to the board?

How do you connect the board to a power source? Which power source?

Your post seems to imply that you connect pc and power source at the same time; do you?