Arduino Uno Rev3 imposible to connect to PC

Hi, I'm using arduino Uno r3. Few days ago, suddenly, my PC couldn't recognize my board! For the record, I'm using win7. When plugged in, green led is on, but connected just with the usb cable , nothing is happening. When I try to upload a sketch I got this error "avrdude: stk500_getsync(): not in sync: resp=0x00", or this one "Serial port COM9 not found" Why's that? Any solutions?

The error message : "stk500_getsync(): not in sync: resp=0x00" means that your Arduino Uno can not be detected, as if it is not there.

It could be broken. Try another usb cable and another usb port. Sometimes it helps to re-write the bootloader into the chip, but I think you have to buy a new Arduino board.

Already tried another USB Cable, updated drivers, but nothing.. As I said, the board isn't dead..but connecting it with the PC it's impossible...

What did you do with it a few days ago?

Well, I’ve tried to upload a sketch for the Ethernet Shield. After that, nothing! Saw this video Using the Arduino Ethernet shield, Part 1 of 2 - YouTube and used most of the code from there…
P.S.: I’m a noob trying to learn, don’t blame me for this

try another uno to check the connections