How to recover access to a lost iPhone's iCloud account?

My sister lost her phone last Saturday. I got a phone call from the phone number and when I call back it only ring and keep blinking. I got a text message saying that the phone was found. And when I open the link and put her email and password the I cloud can’t be open anymore. What should we do? Did they change the password of the phone?

[Re-Titled by Moderator]

Original Title: Lost Iphone