This is something simple that I can't find. When I moved my mouse (cursor position) there used to be a dotted, vertical line that followed the mouse everywhere I went on the timeline (don't know what its called?). But it's gone and I can't get it back. What have I done to make this disappear? Thanks.

Press X to show/hide it. It's called "Aim Assist"

Thanks a lot, Karl. I thought I had probably pressed a key to turn it off but couldn't figure out which one.