I got my uno r3 board a few days ago i used it and it worked fine but today I wired up a lcd screen it still worked but after a while when i tried to upload a new sketch the com port wasn't found (COM 8 )
Please help me this is the seccond board i bought and only one that still works. I can't buy a new one if this one is broken 
Do you see a port for you Uno after you plug its USB cable into your computer?
Sometimes the port will be labeled with the board name in the menu. Other times it will not. If you don’t know which port is your Arduino, you can find it like this:
- Unplug your Arduino board from the computer.
- Tools > Port
- Note the ports, if any, listed in the menu.
- Close the Tools menu.
- Plug your Arduino board into the computer.
- Tools > Port - The new port listed in the menu is your Arduino board.
Nothing shows up
Does the problem still occur after you disconnect all shields, modules, wiring, external circuitry, etc. from your Uno?
Make sure the USB cable is completely plugged in to both the Arduino board and the PC.
You may have a damaged/defective or charge-only USB cable. Try a different cable.
I already tried this and also tried to search in device manager but it didn't show anything
OK, then the USB chip (and likely other components) on the board is dead. If it's one of the Uno's with the socketed ATmega328P chip, you might be able to remove that chip and use it (assuming wasn't also damaged by whatever killed the USB chip), but the rest of the board is essentially garbage.
I quess your right but the chip still works with the last sketch uploaded
I got one more question.
Uploading a sketch using another board as programmer with the RX and TX pins on those will work?
BreinDed:
I quess your right but the chip still works with the last sketch uploaded
Yes, we currently only know that the USB chip on the board is dead. However, it is possible that the conditions that killed the USB chip partially damaged the main microcontroller (ATmega328P), so that it still appears to work, but may have problems in the future. Or it could be perfectly good.
BreinDed:
Uploading a sketch using another board as programmer with the RX and TX pins on those will work?
Probably, though there is a possibility that the conditions that killed the USB chip damaged the ATmega328P in a way that will make it not accept uploads anymore.
You don't even need a whole other Arduino board to do this. You can also use a USB to TTL adapter module (AKA "FTDI"). You can buy them for around $1 USD with free shipping from China on eBay.
Thank you for helping me with my problem. I will buy an usb to ttl adapter and see if it works.
New install of Ububtu. New install of Arduino IDE.
Both successful installs but plugging in UNO, can't set up the COM port. serial ports grayed out. only options are dev/ttyso, or /dev/ttyusbo, neither of which work.
The board works fine with Windows but not Ubuntu.
Please advise. I'm new to Linux so will appreciate any help .