Could be several things. What kind of shape is the wire in? Any chance it has chaffed through and is shorted somewhere between the solenoid and the switch or DC panel?
Do you have a volt meter to test the setup?
If not, try this. Get some spare wire and run from a circuit breaker on your DC panel direct to the solenoid. Turn on the circuit breaker and see if it trips. If so then bad solenoid. Make sure you use a low power
circuit breaker in case there is a short in the solenoid. A large breaker may not trip and you could melt the wire.