After adding filter to plugin’s code, post’s content doesn’t display

Your use of the_content filter is wrong. Please check how it should be properly used in the codex in the link supplied

First of all you are totally replacing the_content with nothing, that is why you get a blank screen. The reason is that you are not calling the $content variable in your function

Secondly, if you’ve called the variable, you would have replaced all content with Test text here as you are overwriting all the content, and not appending it

This should do the job

add_filter('the_content','post_add_me');

function post_add_me($content){
    $text="Test text here";
    $content = $content . $text;

    return $content;
}