Q: My phome won work

I have bought this phone from a young lady for my son. I paid her in full and I went N hgot a sim card added service to the phone by giving my provider the imei number everything was working. Then this activate iPhone popped up an now I can't use it!!! I've tried to contact the lady I bought it from but no answer. I don't understand how I can put a service on the phone n now I can't use it. Please help