It’s not remotely impossible in 2018.
If we seriously wanted to do it - we could easily use existing rocket technology to go there.
It’s not that we “lost” any technology from 1969.
The reason we haven’t gone back is because scientists don’t really need to learn anything new about the moon that would require us to send people there - and there doesn’t seem to be any compelling reason to visit the place.
Going to the moon won’t be cheap - and I think most of us agree that the money would be better spent going to Mars.
The original Moon missions were designed to demonstrate US technical superiority during the Cold War when technical superiority was a strong deterrent to the Soviet Union.
Once demonstrated - any further missions were just PR exercises.
The final moon mission was cancelled - even though all of the equipment had been built already - mostly because the TV ratings had gotten so low that on the last mission, re-runs of the “I Love Lucy” show were getting higher viewership figures than live video of humans walking around on another world!
So we stopped going there.
Subsequently - NASA designed bold new plans to build large modular spaceships in orbit to travel to other places - with an orbital factory facility with 100 people living there. There would be space “tugs” that could move goods and materials around in orbit and haul things up to Lunar orbit. The Saturn V ‘heavy lift’ rocket would be used to get large loads from Earth to the space factory - and a small, lightweight, reusable “space taxi” would take people from Earth to orbit.
Needless to say - the US government trimmed this down a bit. Gone were the modular spacecraft. Scrapped was the space “tug”. Without the modular spacecraft, the space factory could be trimmed from 100 people with vast manufacturing capabilities - to a modest space station with 25 people - then to an even more modest one with 5 people - and then to something that would only be built with support from other countries. Hence the ISS.
The Saturn V was scrapped.
So the only spacecraft left in the plan was the re-usable space taxi…but since it was the ONLY spacecraft left - it had to be made big enough to get the parts of the ISS into orbit - and big enough to cope with various military payloads.
So the lightweight “taxi” got turned into a “truck”…now that it was huge and heavy - it needed solid rocket boosters and a bigger external gas tank - and we got stuck with the ridiculous “Space Shuttle”…which was too big and costly for just moving crew around (so we mostly paid the Russians to use Soyuz) - and too underpowered to get things beyond low-earth orbit - and too expensive to throw away after each mission.
The “reuse” part became a big headline feature - but with a larger, heavier spacecraft with external tanks and solid boosters and cargo doors that could open and close in orbit…re-using it was so costly and so time consuming that it would literally be cheaper to build a new Saturn V for each “construction” mission and a new Gemini-era Titan rocket/capsule for crew operations.
But even that common sense alternative went out of the window because the idea of a “Space Plane” was too compelling…even if very deeply stupid.
So what killed the future of mankind in space for the next 50 to 60 years?
Politics, money, stupidity…the useless ISS and even more useless Space Shuttle.