Green LED does not stay lit once plugged in

I have an Arduino Uno and when I plug it into my computer the green light comes on, but after a second it starts to dim until it turns off. Part of the reason is the normal serial port I use for my Arduino no longer shows up. I have reinstalled the Arduino software and restarted my computer, but the serial port does not appear. I have looked at the "finding serial port" forum questions, but the port does not show up when I go to my device manager in control panel. How can I get the serial port back?