It was EMP related. IIRC, it was explained previously in the film and I believe that there was some special effect to suggest an explosion had taken place in the stratosphere.

By the way, Jason Robard's character did one educational thing: bending (I hope that's the right word) as much as he can behind the dashboard.

Also the movie featured prominently a white noodle-like dust rain that had that charming effect of making people bald.

There was another film with William Hart (can anybody remember the title?) about a post-apocalyptic world where most electronics are fried due to EMP.