Meh, call them on the phone, writing an apology via e-mail or any other medium that does not yield a direct response, where you don't even know if or when they read it, just makes it all worse, you might even write something they will misunderstand yet again etc. blabla.
I know that because I'm apparently good at not getting replys to messages.
The vacuum this leaves is worse than not communicating at all so I usually just choose the latter, for the sake of my own sanity.

Now it's up to you, wait for a face-to-face talk or pick up the phone but my advice is against any other method.