It may have deleted because you accidentally clicked/dragged it off, or possibly because of a localised bit of filesystem corruption. Nothing really to worry about - you can put it back just by navigating to your User folder and dragging Pictures back to its place in the sidebar. They're only links anyway, not actual directories, so deleting or adding them doesn't actually affect anything.
Yeah thanks for the help guys. I was playing around with the console becasue I heard it was similar to Linux and messed some stuff up. ah well tought me not to screw around with my first mac week one lol.