Simplest thing first, check the battery voltage at the + post. If OK, check it at the + clamp to the battery post, if OK, keep your probe on that post, and turn the key to start. If the voltage drops, you have a bad connection between the positive lead and the battery post, or possibly the negative battery connection, or the negative cable to ground.
If the voltage does not drop, then you should check the starter solenoid connection for voltage when you turn the key to start. If there is no voltage there, or very low, it could be the ignition switch, or a poor connection anywhere from it, thru any safety switches and wires to the solenoid.