I cannot upload any new sketch to Bluno on my PC.

The correct steps to upload a sketch using Arduino IDE. Except that, you could also check the possible reasons and solutions below:

1. Driver not installed (Cannot see Arduino UNO(COM*) in Device Manager)

Install the Arduino board driver manually.

2. Driver installed automatically by some BT software (Show up as Bluetooth device but not COM* port in Device Manager)

Uninstall the device and install the Arduino board driver manually.

3. Forget or mistake to select correct Board and Port(Failed to upload sketch)

Select correct Board and Port in Menu bar -> Tools.

4. Serial port was occupied (Error: avrdude: stk500_getsync() attempt 2 of 10: not in sync: resp=0x7d)

Remove the wires on serial port, i.e. Digital pin 0,1, if there were.

If the LED Link was ON, please disconnect the connection between Arduino card and your phone/PC /BLE device, make sure the LED link is OFF.

5. Some inappropriate settings by AT command.

Please read Q1 (above), Step1&2.

6. Bootloader lost or other possible reason

You can get a replacement if it's in warranty. Or you can also try to fix it by yourself. How to flash a new CC2540 chip with DFRobot BLE service?