AutoCAD has two ways of selecting items and performing commands. Either you can start the command, and then select the item you want the command to process – or you can pick an item, and then start the command. AutoCAD will assume that the item you have selected is the one you want to process.
Solution:. In the Layer Properties Manager, make sure the layer that contains the viewport geometry is not turned off or frozen. Go to the layout tab that contains the problem viewport.
From the Properties palette, select the Quick Select button on the top right. On the Quick Select dialog box, set the Object type to Viewport and click OK. If other viewports are in the layout tab, remove them from the selection set by holding down the Shift key and selecting each visible viewport, leaving just the invisible viewport selected. Check the properties and note which layer the viewport is on. Double check that the layer is ON and THAWED.
This may correct the problem if the layer was off or frozen. Move viewport to appropriate layer.Or, if VPCLIP was originally used, then.
With the invisible viewport selected, start the command and type P to draw a polygonal viewport over location of the invisible one. The shape and size don't matter. Start the VPCLIP command again, select the viewport you just created, and use the Delete option. The invisible viewport will now be visible. (You may need to resize and reposition it.).