This problem can occur due to two main reasons.
- Your computer does not recognize your Arduino board
- Your Arduino Board is damaged
Sometimes your computer does not recognize your arduino board. You can check it by opening Device Manager > Other Devices If a device named Arduino Uno is there, probably it is due to the driver issue.
You can solve it by manually configuring the driver from here.
If the Arduino Board is still not recognized, it can be due to a damaged cable or the Arduino board is damaged. Try using a different cable to connect it to your computer. If the board is still not recognized, the Arduino board might be damaged, You may have to repair the damaged board by giving it to a shop or may have to buy a new board which seems the easier way.