It's kind of rare, but there have been occasions where people have had a coax that had a staple poke through it. Almost always, it's a loose, dirty, or corroded connection. If you have a friend with a B&W monitor, that's a quick way to check it. Thrift stores sometimes have 8" B&Ws for $5. Which is lots less expensive than taking it to a dealer for testing.
Also, with a DVM you can measure the coax for shorts. If it has a frequency meter installed, you might be able to read a signal from the camera.
FYI, cameras sometimes die too.