Uno does not respond

I am having trouble with my Arduino Uno. It's been sitting, undisturbed for about 6 months, and now is not recognized by my computer. My duemilanove is instantly recognized, and can run sketches perfectly. When the Uno is plugged in, the power light is on, but Rx and Tx do nothing, and the computer does not recognize anything at all. Is there anything to do to save the Uno, or will i have to keep using the duemilanove?

The computer does or does not make the "device insertion" sound when the Uno is plugged-in?

When the Uno is connected, the computer does absolutely nothing.

Have you tried a different USB cable?

Have you tried a different port on the computer?

Have you tried a different computer?

Using the same cable and port, the duemilanove works perfectly. i'll try another computer and cable if that could possibly be the problem, but i doubt it.

The thing about USB cable connectors is that they wear out. It is possible the cable makes contact with the Duemilanove's socket but not the Uno's socket.

The thing about computers is they can have different drivers installed. If the Uno works on a a different computer the driver is probably the culprit.

When the Uno is plugged-in does the LED on pin 13 flash a few times?

Tried a different cable and a different PC (unfortunately both are running Win7 x64, so i can’t troubleshoot the OS), and got the exact same results. No lights come on except for the steady green power light. no other lights blink or flash.

The Uno is probably defect.
You could use a magnifier to check for problems, or replace the electrolyte capacitors.
Is the ATmega in a socket? That could have bad connections. You could take the ATmega out and place it back.

One more thing to check: Is there 5 volts at the ATmega328 processor? Is there 5 volts at the "5V" pin? Is there 3.3 volts at the "3.3V" pin?

Either the board is "fried" or the two processors have lost their programming. If the little processor "forgets" the Uno is no longer a USB device; which explains the lack of recognition by the computer. If the ATmega328 processor "forgets" the bootloader won't run; which explains the lack of activity on LED 13.

To test the "forgot" theory, use OptiLoader to install a bootloader on the ATmega328. If that works, you may be able to recover the board. If not, it's probably time to recycle.

All of the voltages are correct. going back to school tomorrow, so i don't have time to try the optiboot. Thanks for all your help, its a shame i managed to ruin my Uno, and not the duemilanove.