Check the cables

Inspect the wiring visually, look for pinched, broken or scratched cable sleeves. (unwrap the cable from the sleeve if necessary). Check the cable plugs on both ends by holding the connector with one hand and gently pulling the wire with the other hand to see if the wire doesn't snap out. 

Unplug the cable and see if the connector or its socket isn't damaged. If you have a multimeter, check the cable continuity. See Multimeter usage for more detailed information.

After the check is complete, wrap the cable and plug it back.