What if you no longer have the cable that came with the device?

Buy another cable. It's a standard part made by Apple and many other companies. If you don't get an Apple make sure you get one certified as Made for iPhone.

You can buy a new one, or borrow one from a friend use a different one you may have?

How do you charge the iPhone if you don't have a cable? Can you not use the cable you use for charging?

What cable are you using to charge the phone? Try that one.

Edit: What Phil said.