You can resume by going to the Preview (Alt+P), sliding the slider to the spot where the job stopped, and clicking ‘Start here’ on the bottom. That will re-run the job from that point forward. As to why it’s stopping, there are a variety of reasons this could happen. Make sure the USB cable is secure on both ends, and that your computer is not set to allow USB port sleeping (sometimes called ‘Selective Suspend’).
These are general tips that may help: