Can't think of anything to sketch? Why not put down some lines and add a few markers to them, see what comes up.
It's easy to overthink things. Instead of thinking about the details and overwhelm yourself, start doing and you'll realize it's much easier to go from there.
It's a sketch. Just start moving your hand without thinking about it. The movement of your hand should be ahead of your thoughts.
Don't stop until the entire sheet is filled up with lines, circles, ellipses and ink.
Then do it again.