We've probably all had circumstances change at some point and been faced with the decision to miss out on something we were looking forward to and it can suck.
You were clearly looking forward to the christening, but, this is your grandchild who has been unwell and your daughter asking for help, as opposed to your friend's granddaughter being christened. I definitely think your own family should have been the priority.
Yes, baby had come home from hospital, yes, the husband was arounds, but your daughter asked you to stay, so it's probably made her feel like shit that you didn't.
No doubt your daughter will have found a way to get through the tough time, but now she knows where she stands on your priority list.
I'd say this will take more than a simple apology. As a daughter, I'd have a hard time getting past this. My mother was more than difficult, but had one of our kids been sick, she would have dropped everything to help.