I agree with what everyone else said about hunger, dirty diaper, temperature, etc., so be sure to check all that stuff.
It's hard to tell without knowing how old the baby is. If it's a newborn baby, it could be colic. If the baby is older, it could be that he or she is teething and this is causing his or her ears to hurt. If the baby drinks formula, it could be that baby needs a different type of formula. If baby is nursing, maybe it's something mom has eaten that's upsetting the baby's tummy.
Another thing it could be is the outfit the baby is wearing is rubbing him or her wrong or pinching the baby. Even if you took the outfit off to change the diapers, there could be something uncomfortable about it that you wouldn't notice because you're not wearing it.
If these suggestions given by everyone don't work, try giving baby some ice water in the bottle if he or she drinks from a bottle. My other suggestion is that if all else fails, put baby in the car and take a nice air conditioned drive for a bit.