Try another computer or cable to rule out the cable.
Try another USB port on the PC.
Is this a Windows machine? You may have to reinstall the USB hub. You can find articles by Googling USB device not detected by PC...
Another option if you want to move files between phone and PC but without a cable, would be to go to the Play store and download an FTP server program. Then download an FTP client onto your PC like FileZilla. Then you can drag and drop over WiFi.