By phone would be better, more caring and polite. When you call and ask them how they are doing, how things are in that place, you can roughly understand how they felt when you left without saying. I don’t know about the customs in your place but, if they sound offended by your leaving abruptly, then, you may apologize. That way, you can restart your previous friendship again. Also you can clear off any misunderstandings that arose after your leaving. Good luck.