How do you lock/unlock settings on your iphone?

At some point, I 'locked' the settings on my iphone so that I couldn't add or delete email accounts, enable/disable facetime, etc.

I want to add and delete several accounts but they are 'locked' which means that I can see them on my iPhone, they are active BUT they are grayed out under 'settings>mail' so that they can't be selected to change.

I can't find the setting to unlock the settings so that I can change them.

I know this was to protect the settings in case the phone was lost or stolen, but now I can't remember how do undo it.

Please advise!