galaxy S5 takes a micro sim not nano sim. make sure your sim is fitted in right way and in the right position i.e not where the memory card goes.
if the sim isn't damaged or scratched then your sim reader in the phone has some how damaged or gone faulty and if its the S5 then its gonna be way out of warranty.
if youre using a nano sim in a adaptor to fit in your S5 make sure the sim is flush and fitted in adaptor well.
