Guineas in general are pretty hearty as far as the common poultry diseases go... so it's not common for them to go off feed and water and just give up and die, without an outwardly obvious reason
He may have eaten something toxic, he may have had a high worm load, maybe it was Black Head, he may have hit his head in the coop that did not leave a mark and the internal swelling is what slowly killed him, or he may have had a week heart, failing liver or kidneys etc. Without doing a necropsy on the carcass, it's almost impossible to even guess why he died (and even then you may not have been able to find out the cause).
Sorry for your loss. If you lose another bird this way I highly suggest finding a vet or university to do a necropsy on him to find out if it's some kind of contagious disease or not.