slept through my baby crying

Yesterday my 4 week old ate at 6:30 AM then he went right back to sleep, I put him in his crib set up the baby monitor and went to lay down for a few more hours. I woke up and I could hear him screaming the monitor was buzzing so I ran into his room and picked him up changed him and went in to make a bottle I looked at the time on the clock and it was 1:00pm. He NEVER has slept that long. As I was feeding him his eyes were so red I think because he was crying so long and I never heard him, I slept right through it. I cried because I felt so terrible. This is becoming too overwhelming by myself and I don't know what to do. I don't have family anywhere near me after having to move states for my job a few years ago, when I was 27 weeks pregnant my fiancé left. He's seen our son 2 times since he's been born and never shows up when he says he's going to. I don't know what to do anymore I feel like a terrible mother