mobile

Hi my s5 doesn't show any sd card inserted it just shows the internal when I go to my files,also in storage the mount sd cars is greyed out and says insert sd card I know it's not the card as I've tried it elsewhere,I'm running 5.0 and I'm not sure if it worked before as I bought it from computer shop....I will take it back but would like to know if there's a fix or common fault before I go to the shop many thanks.......