Assuming you mean that the PC just doesn't recognize that a USB device has been plugged in, or that it sees the device but it's unknown, it could be a few things. USB charging cord, not USB data cord. Defective cord. No drivers installed into PC. Plugged into USB hub, not directly into PC USB port. (Some hubs just don't work with some phones.) Defective USB port in PC. Defective microUSB port in phone. Defective phone.