It can have many reasons, but you can try the following:

First time use?

• Check if you need to install a special driver for CH340

Environment/Software problems

• Try another USB port
• Change USB cable
• Restart Arduino IDE
• Restart computer
• Reinstall Arduino IDE

Hardware problems

• Test using another Arduino
• Test using another computer/laptop
• Replace USB cable
• The Arduino is (partly) broken, nothing much to do here

The USB interface chip controls the Rx & Tx lights. I suspect you fried it.Does the '328P feel warm or hot to the touch? That's usually a sign that it has been damaged.

Is it an "official" Arduino purchased from their website? If it isn't, chances are you need to install CH340 drivers for your computer before the Arduino will be recognized. Drivers can be foundhere Have you successfully used this device with your computer before?

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