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 mi** 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 **ween 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.