You didn’t add any code (make sure you add your own code next time because nobody bothers to help you if we have to guess the problem) but Im pretty sure I know what’s wrong.
Straight out of the WordPress codex:
echo get_avatar( $user_id_or_email, $avatar_size, $default_avatar, $alt_text, $args );
You have to echo it because get_avatar() doesn’t output it by default.
Also make sure that “Show Avatars” is checked in Settings because it will not return avatar otherwise.