iPhone unavailable

my iPhone placed on my desk and me not using it, all of the sudden displayed  " iPhone unavailable for five minutes"  it is not password issue at all . I wasn’t using the phone . I wasn’t entering wrong password over and over. Like it says above. It was resting on the table .Happened twice. first time three weeks ago and I tried to restart it wouldn’t let me I tried to take a picture it wouldn’t let me. The second time last week it let me take a picture.and after the five minutes passed, both time,  it becomes available like nothing has happened. Felt like something or someone has taken control of my phone ? Or the phone was protecting itself from being intruded?