Please try these steps and check if the problem still exists:
1. Disconnect the power source from the computer.
2. Remove any External peripherals from the machine.
3. Press and hold the power button for 10-15 seconds and then release.
4. This will release any Flea Power stored in the machine.
5. Re-connect the power source try to power on the computer.
6. Power on the computer and check if the Pen Drive is detected now.
7. If the pen drive is still not detected, please update the Firmware and try using the repair software from the below mentioned links:
8. This tool will help to repair the corrupted 16GB JET USB flash drive with JND Solution. This can be verified by checking on the marking on the USB connector.
Do reply with results.