Apple iPhone 6 Plus A1524 randomly reset to factory defaults

The iPhone 6 Plus in question was my grandmother's and had not been been used in a while. I managed to get in using the password twice with no issues, but when I tried to log in a third time a while later with the same password, it wouldn't work, and the phone wouldn't password lock either. I restarted it, and it opened to being locked for 1 hour. After a while, I went to try and unlock it, but the phone had reset to a white screen, and told me to open the activation lock. Why did this happen, and is all the data on the phone still there, or did it reset itself and wipe everything off?