Access code self-creation

Grannie is 93 years old. Her world is very small and only 2 other people ever touch her iPhone, which also never leaves the house. Her iPhone has always been set up without an access code because she can't remember it. Today, suddenly her phone is asking for an access code. We have no idea how this happened. Any suggestions how we can get back into the phone so she can play solitare would be much appreciated.