I have a list of excuses a mile long, but that doesn't change the fact that I missed a drop-in on my charges. I'm heading over there today, but I still feel like I should be doing something more.
Did you notify the client immediately? Since you didn't give any more information, it's hard to tell what to do. First thing would be to notify the owner immediately and offer some remedy to fix the situation.
It would be very different if you were to drop in on a cat, and missed a visit. But if you were to take care of a dog, and didn't show up at all, and you are going to be going there today, then that's a problem. Either way, even if you miss a visit, try to make it out there later, clients will understand if you are running late, but they won't forgive you if you just don't show up and don't even bother to notify them.
I hope you can sort it out. Good luck.
You should: have told the owner immediately when you realized you missed a drop in. If you did, and they still want you as a sitter (since you went again) it seems to be okay with them.