I just updated my I-Phone & i am not able to purchase music on I-Tunes, so what could be the issue

I just updated my I-Phone to the latest software & i am not able to download music . I push the icon to purchase & the security password never pops up, for me to put in & it never downloads, or purchases. Can anyone tell me as to why its not working, or if your having a similiar problem, or if its a problem on I-Tunes side