Babies that are 6 months of age or younger are usually not able to roll back over onto their backs so if the baby does roll over onto their sides or stomach while sleeping and you notice it then you should put the baby back on it's back to reduce the risk of SIDS.
Once the baby gets above 6 months of age or older then the baby can and should be able to roll themselves back onto their back from their side and stomach.
Avoid putting anything soft into the crib with the baby to avoid suffocation until the baby is above 6 months of age and preferably wait until the baby is at least a year old just to be safe.