Can Sprint lock my iphone if I haven't paid the bill?

I have an iPhone 5s from Sprint, my husband is recently fresh out of the Navy and we have had a hard time getting back on our feet. My sprint bill was a month and a half late and was just turned off. My husband needed a phone so he got one through Verizon. I went in to Verizon with him today and was told they would take my Sprint phone through trade in -- long story short, my husband was late for work and I never got my Verizon phone or traded in my iPhone.

While at the Verizon store they had me remove my passcode from my phone because they needed to access it when I traded it in. I have been home for like 2 hours and suddenly my iPhone has a passcode on it again -- my old passcode isn't working and I can't use the fingerprint scanner on it and I can't even use it to call Sprint to ask them if they put a code on it. I don't know if they can even do that, hence why I'm asking. The phone itself isn't on some kind of "payment plan", I bought it out right and it's paid in full. I had missed paying my bill once before about 2 years ago with my iPhone 4s and they never put a passcode on my phone. I know it sounds like a dumb question but this has never happened to me before! The last and ONLY passcode that has ever been on this phone doesn't work. Any ideas?

Added (1). that's what I thought, I never heard of them putting locks on phones, I just figured my service (phone calls and texts) wouldn't work. I don't have much on this phone, I had to factory reset it like a week after I got it and never added much to it because I haven't had time with our move… So here's another dumb question, if I reset the phone and delete everything off of it, now that I don't have service, could I still set a passcode of my own on it? I wouldn't want someone being able to access the phone and sell it if someone were to steal it and try to make a profit (worse case scenario, I know, I'm just paranoid even if it is really just an expensive useless brick right now!). We're living right outside detroit LOL and I'm not used to people being so cruel and jacking other people's stuff all the time. I'm sure I won't bring the phone with me much but since I still have the ability to use it as an emergency phone I might. Better saf

There isn't a way to bypass it, but to factory restore it using iTunes. I have a feeling the store owner did that, you should go back to them and ask if they put one of theirs on it. That seems quite dubious and never have I heard of a carrier "locking" a phone due to a non-paid bill.

No Sprint or other carriers can't lock your device and prevent you from using it. All they can do is bar your service so you can't make /receive calls. You may have to reset your phone but do this at the last option: http://osxdaily.com/2011/01/16/forgot-iphone-passcode-how-to-reset/

Sprint can indeed "blacklist" your phone for payment owed, then your phone will be useless on sprint or any other mobile carrier. Your phones ESN is flagged as "bad", search for the term "bad esn", it means you will never make another call on your phone again.