When I open up a camera app only the front camera shows a purple line

I had just recently bought my iPhone 7 (December 2018). I have not cracked or dropped it in any water. But when I open up a camera app only the front camera shows a purple line. And I only found around 7:00 at night. But when I checked my phone and took at picture at 9:00 this morning it wasn’t there. Why is there a line?

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