Since you don't know the reason for not getting a response from the guy or girl you've been dating, it's a good idea to be polite in your email. After all, he may write you back with a legitimate reason for not having called you in a while. By formulating an email that has a polite tone, you are maintaining the peace, and not causing unnecessary drama.
Proper email etiquette includes being brief. Rather than rambling on and on in your email, telling your life story or sharing all of your feelings, make your email succinct and to the point. This will make it easier for the recipient to read, and she'll appreciate an email that isn't the length of a novel.
Greetings and Closings
It is proper email etiquette to address the person in an email with an appropriate greeting, even if you are angry with him for being out of touch. If the email is being written in the morning, say "Good morning." Closing the email is another proper feature to include. Saying "Goodbye" or any other form of signing off from an email dialogue shows respect to the recipient.
Again, you don't know why your date hasn't followed up with you, so it's not in your best interest to shake your finger in her face. The content of your email can be suggestive and symbolic of how you are feeling, so try to make sure that your fingers don't type everything that is going on in your head. Even though this can be emotionally taxing, it's proper etiquette to give the other person the opportunity to explain himself.
Besides tone and content, follow-up emails can also get an emotional indication across through the email's format. Using all CAPS, for instance, is considered yelling in email culture. Even if you feel like yelling at the other person, it's not appropriate etiquette to make major emphases that the other person can interpret as shouting.