For example, if there are no monsters on the field, you cannot activate "Ceasefire" because it will not do anything.
However, if there is a face-down monster on the field, you can activate "Ceasefire" because it will flip the monster face-up.
You can also activate "Ceasefire" even if there are no face-down monsters on the field, as long as there is at least 1 face-up Effect Monster on the field. This is because "Ceasefire" will do 500 points of damage to your opponent for each Effect Monster on the field. So even though "Ceasefire" will not flip any monsters face-up, it will still do damage, so you can activate the card because it will have an effect.