Shut off and now will not crank or start

Hello, sorry to hear of your troubles. Do you own a multimeter to do some electrical checks? Or an automotive test light to check for power. Below are the wiring diagrams for the Starter Motor circuit. So, your car just shut off when you turned the heater on? The 3rd diagram is from Chevrolet. It is the original wiring diagram, underlined in yellow are Fusible Links, they are down by the Starter Motor on this model if I remember correctly. These are just like fuses; except they are a section of wire that is made to burn through if there is a surge of current flow. Since you have checked all your fuses, I would check these next.
It can be difficult to tell if they are burned through because it happens on the inside of the wire. You can see that the power wire is Red in color and the Fusible links are a thicker section of wire and a different color.
You may have to piece the other side of the Fusible link to see if there is power there or not. I would shut off the heater first, because there may be something in that circuit that is shorted out and caused this problem to begin with.
You can see above the Fusible links on the diagram, it says the power goes to the Blower controls from there. So, I think down by the Starter where these Links are is where you'll find the problem.

It's possible the Blower motor shorted out or something else on that circuit, so I would leave that off for now after you get the car started again. Until you have time to look into that issue. Finding a short to ground can be time consuming,

https://www.2carpros.com/articles/how-to-check-a-car-fuse

https://www.2carpros.com/articles/how-to-use-a-test-light-circuit-tester

https://www.2carpros.com/articles/how-to-use-a-voltmeter