Here are your parts
Parts for Whirlpool RF315PXEW0 Range - AppliancePartsPros.comSee the attachment for the tech sheet.
Check the power to the unit first.
Try flipping the breaker off/on slowly a couple times. Sometimes you can loose half the line without actually tripping the breaker.
Check the voltage at the plug
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.
Do this with the heater off and on.
[COLOR="Red"]Be careful as 240 volts is lethal !!![/COLOR]
I am assuming that the unit stays in preheat and never comes ready.
Also that the wiring is OK.
If yes to all the above then the control board is at the top of the list.
Unfortunately it looks like the control board is obsolete but you will have to call the parts representatives to confirm this.