I agree with Bruce but suggest you start a bit more basic. Remove cables from battery. Clean battery terminals and connectors thoroughly. This could even be a good time to have the battery load tested. Reinstall cables on battery and tighten. If you can move them they're too loose. Follow ground lead to frame. Clean and tighten securely. I'd start the actual troubleshooting knowing battery and connections are good. Then again, if the problem is solved no further troubleshooting is required.