They always say a convoy is only as fast as the slowest vehicle. Likewise, your photoshoot is only as strong as the weakest link the sync cord. This $15 cable can cause more photoshoots to grind to halt than any piece of equipment.
The sync (short for synchronize) has an incredibly thin wire running through it, and the PC (Prontor Compur) tip that connects to the camera or radio slave is a weak design (but the industry standard). The PC end has a pin and collar around it that connects the circuit and fires the flash. The pin and the collar can be become damaged and cause the flash not to fire.
To find out the sync cord is broken, take a metal object (I usually use a key) and touch the pin and the collar with it at the same time. On a working sync cord, it will fire the flash (no! it will NOT shock you). Several companies make a PC tip conditioner that will straighten and repair the pin and collar that make up a the PC tip. One type of end that plugs into the flash is known as household and looks likes a two pronged electric plug. Yes, I have known people who have fried 4×5 shutters plugging them into AC. The newer standard has become the mono plug. They look like guitar or ear phone jack. Whichever you need, buy a couple and keep them handy. Happy shooting.