Hi
My UNO has burned out(okay, it was 6 months ago).
I hooked it up to a LCD and connected it up correctly.
The LCD wasn't displaying anything, so I connected up the backlight and forgot the resistor.
The ON led turned off and the polyfuse started getting really hot.
Since then, I have tried many different things, such as powering thru VIN, USB, 5V, and lately
cutting off zener diodes and the 5V regulator.
Nothing helps, and the ON LED does nothing.
The processor is burned as well, and when I put it in my other UNO(a counterfeit), it just gets hot.
When I power up the board without the chip, the polyfuse heats up.
The fuse only gets hot when I power it with USB, not with the power jack or VIN.
Please help!
TIA
Isaac
Time to throw it in the garbage.
The more equipment you ruin the more experience you get 
If your processor is removable, test its pins for resistance to ground, if you are lucky you may only find 1 bad i/o pin which you can bend up to bypass, if you are unlucky there will be an internal short which kills the chip.
With no microprocessor installed, try to repair the damage you caused while 'trying to make it work' if you put more than 5v on the 5v input, then you probbably burnt the USB chip also. A rule of thumb, if something gets hot there is a problem, find out why. With no microprocessor installed does something get hot on you uno board? If your other UNO works, continue having fun learning.
I tried that, 5V is shorted to ground. Also, I removed the regulator but nothing changed. I suspect the 16u2 chip or maybe the power-selection circuit.