...no clue...what happened?

With a cactus?

...Throw a pie at them?

I am terrible with advice.

You wait until they hear you out.

hmmm keep trying unil they listen i'd say or give it sometime then try again or ummmmm wait it out?

I say keep trying to apologize. If they don't right away, wait a bit then try again.

write them a note and slip it under their door.

go up to them face to face and sit down with them...do not take no for an answer, ask to at least be heard out and if they still wont listen, give them some space and try again after they've had time to calm down...also i've been worried about you, you didnt respond to my last note and you havent been around for months

also, you dont want to try repetitively or it'll just make matters worse, trying to rush things will only stack the tension faster. a good practice is writing everything out, every event you've had together and every mistake that has been made, then read it out laud to yourself. think at everything that has happened and everything that could have lead to this moment, often it's an accumulation of many small events. stay calm and make sure they stay calm and listen, dont make things seem like it's their fault but dont make it seem like it's entirely your fault either, the key to an apology is balancing what needs to be heard and what one does not want to hear

I don't know, but it is good to ask! I have the exact same problem with a friend of mine on FA. At least I hope he is still my friend.

A good question. But there also gets to a point where it's too late to apologize. (No pun intended I swear, although I did think that's what this was in reference to to begin with.) Some people just can't be helped though, no matter what you do.