Why would a baby not stop crying?

Try cradling the baby for a while; give them something to 'eat' or drink; check their nappy and change if necessary. If nothing helps, check their temperature and see if they have a rash of any kind on their body - these can indicate ill health or something that would irritate them. If all seems well and nothing has helped, you may wish to take them to the doctor for a checkup: nothing is not worth an appointment for them.