Looks like you are using this theme: Atmosphere 2010

Looking through the theme files the function atmosphere_posted_on is used a couple times in loop.php and single.php. The undefined function error indicates that the function does not exist which leads me to believe that your functions.php has been modified or is otherwise missing this function. In the version I downloaded, the function looks like this in functions.php starting at line 281:

if ( ! function_exists( 'atmosphere_posted_on' ) ) :

function atmosphere_posted_on() {
    printf( __( '<span class="https://wordpress.stackexchange.com/questions/22228/%1$s">Posted on</span> %2$s <span class="meta-sep">by</span> %3$s', 'atmosphere' ),
    'meta-prep meta-prep-author',sprintf( '<a href="https://wordpress.stackexchange.com/questions/22228/%1$s" title="%2$s" rel="bookmark"><span class="entry-date">%3$s</span></a>',
        get_permalink(),esc_attr( get_the_time() ),
        get_the_date()),
        sprintf( '<span class="author vcard"><a class="url fn n" href="https://wordpress.stackexchange.com/questions/22228/%1$s" title="%2$s">%3$s</a></span>',
            get_author_posts_url( get_the_author_meta( 'ID' ) ),
            sprintf( esc_attr__( 'View all posts by %s', 'atmosphere' ), get_the_author() ),
        get_the_author()));
}
endif;

You might try uploading a new version of functions.php or see if your modified version still contains this function.