A thin line / stripe appeared on my iMac's screen

A thin black/ dark blue vertical line / stripe appeared on my iMac's screen today, on the left side.

The line is quite blurry. I think it happened when I opened a movie in the VLC media player.

That is when I first noticed it. Does anyone know how I can get rid of it?

On this ''screenshot'' you can see how it looks: (you have to click on the photo to see it)

I drew the line on this ''screenshot'' myself because I didn't appear on a actual screenshot.

If anyone could help me I would be very grateful..