I made a shape with bezier curves, then converted it to a mesh, then extruded it along one axis. I can't remember which end is the original at this point. I selected all the vertices on one end and use F (in Edit Mode) to make a face with all the selected vertices. Then I went to the other end and selected all the vertices on that end to do the same thing. When I press F, something happens, but it's hard to see just what. It looks like when I press F to add the face, that some of the lines that run from end to end change to orange and when I undo that, they change back. Even when I've zoomed in, it's hard to see just what is happening.
I select the vertices in x-ray mode, to be sure I get them all, then I change to solid mode before creating the face, so I'll see the new face.