This question doesn't really seem to be related to Newton directly.
Perhaps, questions of this nature should be posted in the 'scripting' forum category instead.
red: removed element
blue: pay attention to
Entity pointers might be reset to null on level_load, but while(1) functions may 'persist' or continue.
Code:

---

 starter move_camera() {
	//proc_kill(4);		//only one instance of this function should be running
	camera.arc=72;
	camera.clip_near=20;
	camera.clip_far=24000;

	var cam_ang[3];
	var cam_dist=500;
	cam_ang.pan=180;
	cam_ang.tilt=18;
	cam_ang.roll=0;

	//while(!p_vehicle){wait(1);} // removed; probably unnecessary

	while(1) {
		if (p_vehicle != null) {
   			if(key_b) {
	 			cam_ang.pan = p_vehicle.pan - 0;
			} else {
	 			cam_ang.pan = p_vehicle.pan - 180;
			}
   			//{ //dangling bracket removed

			if(mouse_right) {
				//cam_angA.pan+=10*mouse_force.x;
				cam_ang.tilt=clamp(cam_ang.tilt+(10*mouse_force.y),5,80);
			}
		
			cam_dist=clamp(cam_dist-mickey.z,200,2000);

			camera.x=p_vehicle.x+cos(cam_ang.pan)*(cam_dist*cos(cam_ang.tilt));
			camera.y=p_vehicle.y+sin(cam_ang.pan)*(cam_dist*cos(cam_ang.tilt));
			camera.z=p_vehicle.z+sin(cam_ang.tilt)*cam_dist;

			vec_set(temp,p_vehicle.x);
			vec_sub(temp,camera.x);
			vec_to_angle(camera.pan,temp);
		}
		wait(1);
	}
}

function reset_level() {
	
	//dll_handle = newtonHandle;
	//NewtonRemoveMap();				//or maybe instead
	//NewtonRemoveAllBodies();		
	
	loadingscreen.pos_x = (screen_size.x - bmap_width(loading_lv))/2;  //centers loading screen
	loadingscreen.pos_y = (screen_size.y - bmap_height(loading_lv))/2;
	loadingscreen.visible = on;  //displays loading screen	
	wait(3);
	sleep(2);	//TD: why are we sleeping?
	loadingscreen.visible = off;  //clears screen
	bmap_purge(loading_lv); 
	LOAD_LEVEL (s_level);  //level name 
	dll_handle=newtonHandle;
	NewtonAddMap(s_level,0);
}
//on_t = reset_level(); //the game is going to be restarted 
on_t = reset_level;	 	//TD: only use parenthesis when actually calling function

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edited: Moved wait(1) into while(1) block.