Cried a lot

Hi everyone,
I feel guilty. My daughter has been sleeping in her own room for a week now, she's 7 months old.
I have the baby monitor in our bedroom. My husband sometimes sleeps in the guest room because he has to get up very early for work and in that way he doesn't have to deal with the night feeds either. That's what happened last night too.
Our baby monitor beeps for a while when the batteries start to run low and you have to charge it, but it usually lasts a day or two. So last night I fell asleep and woke up at 04:45! My daughter usually comes for a feed around 3:00. The baby monitor was off! I don't understand anything about this. Did I sleep through the beeps? I can't imagine it, I'm such a light sleeper. I heard her crying very muffled because her door was closed.
She was lying crying in her crib, the mattress soaked with tears. I don't know how long she had been crying, but she usually comes precisely at 3:00.. so probably almost two hours . I'm so scared this will have an effect on her. She's quite out of sorts today, usually she's cheerful and looks at me a lot. Today she's mostly just staring around.

Of course I'll prevent this in the future. Leave her door ajar, my door open, the baby monitor always in the charger.. But almost 2 hours of crying is so long! I usually try to let her cry as little as possible. Can someone reassure me?