What will happen if i let my baby cry?

Depends how old?: Children under the age of 12-15 mo really can't "cry it out". That said, you can let them cry 5-10 minutes before checking them out. At first, when under 2-4 mo, you should be there fairly quickly, within 5 min or so. Don't fall out of the shower, but come as soon as you can. Babies lack object permanence, so when you're gone they get upset. For older babies, they should see you every 20 min or so.