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If you were using AT&T on it, then you have to file an unlock request from here --> https://www.att.com/deviceunlock/#/
You have to fill the unlock form and send to AT&T. It might take a couple of days to get it fully unlocked. Once it is unlocked (you will get confirmation), you may need to activate it from iTunes on your computer.
So download and install iTunes on your computer from here : http://www.apple.com/itunes/download/
Then just connect the iPhone to your computer and you will get iPhone unlocked message. If not, then select your iPhone in iTunes at the top left. Click on "Backup Now" button in iTunes (Make sure "To This Computer" is selected). Then click Restore button and this will restore the iPhone to factory settings. After that it will be unlocked and you can restore the backup you made earlier to it so you will get your files back.
See here --> https://support.apple.com/en-in/HT201328
Remember, only AT&T can unlock your iPhone legally and it would take a couple of days to process your unlock request.
If you have any questions or would like any additional information, please don't hesitate to ask.