Hi,
I cant get my arduino Leonardo to work 
When plugged into PC These 3 lights stay on, It is not detected by computer
I have tried it on several devices but still not working
I'm afraid it is broken =(
If you have a programmer, you could try to rewrite the bootloader using the ICSP header.
I have a simular issue that I just had. I plugged into my IDE and uploaded my first sketch with success. I pulled it off afterwards and powered it up with the 12vdc power plug and the "on" LED was the only light on and the ATmega32U4 got really hot.
Tried plugging in to the USB after it cooled, and nothing.
I think its fried.
What 12V power plug did you use? Did it come with that? I didn't think they came with power plugs. I always use 9V batteries.
Plus if you are just tinkering with the Arduino, I think it's best that you do it with a battery and not while it's plugged into the wall. If you short something out or what ever you can get seriously hurt from being plugged into the wall.
I was always told to use 9V batteries when experimenting because they can't really hurt you in any way.