I'm a noob. While following a tutorial on using Spin, starting by creating vertices in Edit Mode, somehow my object's verts got offset from the Z axis. I had a devil of a time moving them back because I couldn't simply find anywhere to type in numbers to tell Blender to move the object's origin to 0,0,0. Where can I see and set any object's exact, absolute position?
In the 3D Viewer, press n to bring up the side-menu with panels you can use to edit the position, scale, and rotation of vertices and objects. Also, a quick fix to your problem would be to press A to select all the vertices, then press s, z, then 0.