Content won’t load when using action the_content

the_content is not an action, it is a filter. You should be concatenating a string and returning it. It sounds like you are echoing content directly.

That is:

// wrong
function test_content($content) {
  echo 'this is wrong';
}
add_filter('the_content','test_content');

// right
function test_content($content) {
  return 'this is right '.$content;
}
add_filter('the_content','test_content');