Hi Mark - you're talking about this curve here?



One possible way to surface a closed curve like that is to split it into 2 halves using the Edit > Trim command (click the "Add trim points" button inside of the Trim command to set up a couple of cutting points on the curve). Once the curve is in 2 separate pieces like this:



It is then possible to use the Loft command to make a surface between them.

However, even though that is one way I don't recommend doing it that way because it makes a surface that is kind of collapsed down and pinched together where the ends are touching - it's also pretty easy for the surface to be slightly self-intersecting in those areas too.

So instead the best way to make a fill surface for that curve is actually a pretty similar process to what we were talking about before with the booleans, which is to focus on the original 2D profile curve of the shape, and use that to build a fully extended surface and then trim away the parts you don't want.

Note that when you view that closed curve from the side, it has a nice 2D profile:



So that means that you can build an extruded surface like this:





Note that the closed curve you want to surface actually resides on that extruded surface.

Now you can select the full extruded surface, and use the Edit > Trim command to cut it with that curve, to remove the outer portion that you don't want:



This kind of construction is generally much preferable to the 2-halves being lofted method because the surface has no pinching in it - if you turn on control points for the trimmed piece, note that the underlying surface is still the nice and clean extruded surface:




So somewhere you probably have the original 2D profile curve used to build that solid by cutting its ends off. That's the one that you want to extrude to get the base surface to trim.

I've attached a filled in version here as filled_curve.3dm - I was able to extract the profile curve from this face of your solid, which is where I think you copied that closed curve off of:



I used Edit > Separate on that and then turned on control points and saw that the underlying surface was an extended extrusion like this:



So working off of that surface separated from the main solid, like this:



I selected the edge that is the closed curve that you wanted to fill (I think this is the right one?):



Then I did Ctrl+C to copy the curve of that edge to the clipboard, we will want it again in a moment. Then do Ctrl+A to select all edges and then push Delete - that will do an "untrim" and remove all those trimming boundaries and recover the full original underlying surface. Now do Ctrl+V to paste in the curve we copied, and you have this:



Now you have a surface and a curve on that surface which you can use to trim it with:



But presumably you would somewhere have the original 2D curve you used to build that area of the solid to start with - if you do you don't need to do these separating and untrimming steps here, just go to that curve and extrude it to build the base surface that you then need to trim.

Hope this helps!

- Michael