Hello, we just purchased a used GE electric cooktop and everything works but the heat on all burners. When I turn the knobs, the red light goes on, but not heating??? Any advice would be greatly appreciated.

Most likely a power problem.

The indicator lights run off of 120 volts but the elements themselves require the full 240 volts (L1 to L2).

Try flipping the breaker off/on slowly, sometimes you can loose half the line without actually tripping the breaker.
If this does nothing, check the voltage at the plug. Your unit is probably hard wired so you have to check it at the terminal strip.
L1 to L2 should be 240 volts
L1 to Neutral and L2 to Neutral, both should be 120 volts.
If OK
Unplug the unit and check the wires at the terminal strip in the machine to make sure none are loose or burned out
If OK
Check the power at the terminal strip.
[COLOR="Red"]Be careful as 240 volts is lethal !!! [/COLOR]

You are welcome.

And thanks for getting back to us. Now when others search for a similar problem they will see what actually worked instead of just suggestions about what could be the cause.