hi my phone will not activate i have a virgin sim that did work fine until my daughter connected my phone to the pc and now says there is no sim

hi before my dad passed away last year he brought me an i phone 3gs he got it so i could use my virgin sim in it so i didnt have to change my number my brother connected it to his laptop and now it wont work it says there is no sim in it, i have taken the sim out and tried putting it back in and it is still saying the same thing i dont know what to do, i am really upset cos my dad got it for me, i dont know if my brother did a update or what plz help