Where have all my older photos gone in iCloud?

I have recently purchased an iPhone 6s Plus. Before setting up my phone, I checked that all my photos were stored in iCloud (some of these I had transferred over from a PC a long time ago). Now all my photos appear to have vanished into thin air and there does not appear to be any reference material under apple support as to what has happened, why or how to retrieve. The support information is just taking me round in circles thus very unhelpful.