The technical ability to go to the moon, or even low earth orbit, is at our finger tips. The practical ability to do so today does not exist in the NASA storehouses.
The mathematics required to go to the moon and return was at least half the battle. Anyone who has had to slog through Battin knows that pain. But we are, to a certain extent, beyond that now. Our ability to simulate orbital mechanics and transfers far exceeds anything imaginable back in the last 50s and early 60s. NASA didn't not land rockets back on earth like SpaceX because they didn't think it would be more convenient, they didn't do it because the entire computational infrastructure that existed couldn't handle the mechanics.
Just about everything that was done has been advanced since the Apollo era. Will we need to re-invent some things? Sure, but in many cases the materials, technologies, and capabilities we have today would make all but the lessons learned books* obsolete for new construction.
We haven't really "lost" anything but the will. And by will, I mean solid, long-term funding commitments.
*yes - they do exist. They have been written for many missions and you can browse through them at several NASA libraries.