Extracting line from poly...

You won't be able to physically turn the poly object back into a spline but you can extract the spline shape from it. Here's how:

In the same editable poly menu, select the 'Border' tool and click on the object. You will be selecting all of the open edges. Once done, scroll down a bit in the same menu and select 'Create Shape From Selection'. This will extract the edges you've selected to a spline shape.

Since the extrude didn't work as expected, I think you extruded with an open spline. To fix this next time, make sure that the spline you want to extrude is closed (it will tell you either open or closed by selecting the entire spline shape in the edit menu).