I slapped my baby daughter tonight?

I had a really long day, lot of bad stuff happend then my wife told me she had had enough and stopmed out of the house so i was stuck with our daughter and she wouldn't stop crying. i rocked her, snag to her, fed her, diapered her, everything and then i just threw her down on her crib, she began to cry harder so then i reached in and slapped her as hard as i could across the face and now she's really crying, but my day was terrible and it won't happen again. so what should i do? she's 4 months old and i don't really know, i'm just letting her cry it out right now.