My phone recently locked me out and says 'locked sim' and now requires a PUK pin number I took the phone to AT&T and they told me ''i dont know why its doing that'' can someone help me? I also bought another sim card thinking it went out and same

My phone recently locked me out and says 'locked sim' and now requires a PUK pin number I took the phone to AT&T and they told me ''i dont know why its doing that'' can someone help me? I also bought another sim card thinking it went out and same thing so AT&T told me to put my orginal sim card back in the phone and it will some time and I have no idea how it lets me make a call but i can use it one second and next it be locked out...