Arduino does not work
It's normal that the L-LED is steady on after the USB connection is established. However, after pressing and releasing Reset it should flicker for a second, then go back to on again.

Check https://www.arduino.cc/en/Guide/Troubleshooting#toc1 for reasons why you can't upload.

On http://www.arduino.org/products/boards/4-arduino-boards/arduino-uno I've found:


  The Arduino Uno has a resettable polyfuse that protects your computer's USB ports from shorts and overcurrent. Although most computers provide their own internal protection, the fuse provides an extra layer of protection. If more than 500 mA is applied to the USB port, the fuse will automatically break the connection until the short or overload is removed.


I haven't found out so far, how to read the current values of the fuses.