Unfair right up?

I was written up at work for not showing up, I made sure to let my supervisor know that I wasn't able to come in due to a family situation I needed to be there for. I showed up the next day and he asked me more about it and said he'd take my word for it. Then Im leaving and am in my car and he texted me that he's going to write me up tomorrow for the day I missed? I find it completely unfair, I missed that day due to a family situation and he just suddenly changed his mind after saying he wouldn't write me up. What should I do? (I'm a permanent hire, just newer)