Straight Talk is saying I have no service?

I have an iPhone 5s. My service end date is supposed to be September 29th, but service/data stopped working a few days ago. Customer service is no help at all. They're telling me it's been deactivated. Why the hell would it just stop working?

Is this a Straight Talk phone or a phone that was Locked to another Provider, then Unlocked to use on Straight Talk?

Also did you buy the phone second hand?

If the answer to these two questions is yes, I have a feeling I know what happened.

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