What does ?x show? What other definitions have you made?
This seems likely to be a bug, but I expect you would have to report enough details to WRI (through the Help > Give Feedback menu command) for their developers to be able to figure it out. For me the code plots a line, as expected.
If you can't figure out how the trouble was created, then restarting the kernel (use Quit[] for instance) should reset the system.
