Arduino Leonardo stopped working

Hi,

We have a few arduino leonardos - two of which have stopped working only a few days after beginning using them. We power on the board using USB (with nothing else connected) and the lights "L" and "ON" come on, but there is no activity on TX or RX and nothing shows up on the computer. It worked fine with these computers before (tried on a Linux laptop and a Windows laptop) and nothing shows up anymore. Another leonardo board shows up just fine, but this one no longer works.

How can I fix this?