I need desperate help

I seriously messed up and don't know what to do. My best friend called me asking if I'm free (to talk). I'm on vacation and I was in a place to eat with my family. I don't know why did I just into saying no instead of ,,Yes, just give me a second”. As they were hanging up I asked is everything okay, then right after the call I asked again and said how they can talk about it now aka that I'm fully avalible now. They said that I'm too late. I said that I'm sorry and that if later they may want to talk about what happened, that they feel free to do it. Later I asked if they're home and they ranted about me not answering. I apologised and said what happened aka what I did wrong. How me being on a vacation doesn't excuse it and how it shouldn't be a reason to put feelings and troubles aside. They said how nobody is ever there for them when they need it and similar stuff. At the end I said that I understand if they don't want to, but to feel free to still reach out to me if they ever need and actually want to, and to take care of themselves.
Which I know won't happen in a while because I know I completely ruined everything and broke their trust now. I really hate myself even more for everything I did. I'm not saying that to make me the victim because I'm absolutely not even close but I really do, I’ve been crying for hours.
Generally, when they want to vent to me, I reply right away and go through everything. I remember them sending videos indicating it actually did help them. But this time was when they needed me the most, even more then all of these vents.
I'm so scared that they'll do something to themselves and I'll have no chance of even stopping it. I seriously don't know what to do. I understand it will take a lot of time to rebuild trust but I'm just too scared that they'll do something to themselves. I don't know what to do.