It sounds like your UCS is rotated so that the Z-axis is now "vertical" relative to your screen.
Are you working in 2D, or 3D?
If 2D, then type UCS > World at the command line, followed by PLAN > World and AutoCAD will be reset to it's default UCS and PLAN settings. If you're working in 3D, you'll probably want to spend some time reading about how to manipulate the UCS, but the easiest method is to use the View toolbar, or the View Ribbon panel as they can rotate your UCS to match the view.
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