Hello: Let’s start from the upper panel on the heater. You should find 240 volts above the limit switch. If it isn’t there, back up to the wire nut connections in the junction box on top of the heater. Do you find 240 at the incoming wires? Next. back up further to the breakers themselves and measure at the screws where your two hot wires hook up. You should find 240. If you find no voltage between the breakers, they are both on the same leg of power or one has failed.
This is beginning to sound like it’s either at the connection to the heater or limit switch. Do let us know what voltage you find at the various places 😉