Were you holding down the fire button? The player will remain still when firing, and the move controls become the aiming controls. Releasing the fire button should let you move.
The fire button is either the space bar (or enter) on keyboard or the A/B button on gamepad (you can check the README.txt file). It might be that the space bar or one of the buttons is stuck? Do you have a gamepad connected to your PC while playing?
