That means the phone has been network-locked. You have typed a wrong PIN or PUK many times, and because of that it has been completely locked, but not broken. In some cases, this is considered as being "soft-bricked". To fix it, you need to visit a local phone service shop. They have custom hardware to connect the phone to in order to remove the lock. The price ranges based on the type of the phone, its popularity and difficulty. Unfortunately, this is the only way to do this.