Previous owner must have used find my iPhone to disable my new “used” iPhone. What can I do?

I bought a used IPhone 6 plus online. No interaction with a previous user happened and I have no idea who previously owned the phone. I was able to use the phone no problem for quite some time, Then one day I was as asked to enter the apple user ID and password. When entering my info it said it was incorrect. The only thing I can figure is that the previous user made it not work through the “Find My IPhone” function. I’ve reset the device but still have to enter info that I do not have. Is there a work around?