My phone wont find the network.

Hi i would be really grateful if someone could help me with what is wrong with my iphone 4s it wont find the network I am with, it keeps saying searching, I sent it off to the applestore to see if they could fix it but they said they couldnt as I got my charging docket fixed my someone else, now I got the phone back to set it up again but every time I put my sim card in it says please inset sim card and my sim card works in other phones I have tried. I would be so happy if someone could help me as the phone is only 4 months old. does anyone have an answer for me please.. thank you shauna