Not an expert on this but will try and help, this may appear to be an easy job, is it one ridge or more? if only one it may be easly cemented back in place as said, but beware if one has been removed by wind it may be that all are a bit loose, ( hope not) if an old house it may be that the mortar has deteriorated, and it would be better to have them all inspected , by a builder , have you house insurance? if so you need to contact them , thay will then ask you to get quotes for repair. also be cautious if you do decide on a D.I.Y you could do more damage to your roof tiles. hope this helps some but may be a builder wil see this and give you some more advice.