Technology gets lost all the time, even in modern times. There was a crystal laser which used to be made, doped with some element I forget off hand. The laser can no longer be made because the technology to make it was lost. That one was due to the head researcher not quite writing down all the details in the manufacturing of the crystal and when that guy died. the new guys came in and couldn't recreate the procedure to how the crystal was made in such a way as to allow lasing. Mostly things are lost due to improper documentation. How this specific one (the lunar lander) was lost was because after the Apollo missions were done, everything was disassembled and stored. The industries which made everything were disassembled and we no longer have what it takes to get to the moon. We would have to start over, and starting over in an era of different technologies and safety requirements. The old style manner of getting the moon and landing people on it is gone though and we no longer have the ability to go there.