Concrete wasn't lost per se; the technology of modern concrete is significantly different from the Roman variety which owed much of it's strength to local volcanic ash. This is similar to 'Damascus Steel' which owed much of it's strength and quality not to technique but quality of supplies which, when they dried up, made further forging impossible. The techniques and knowledge of the use of these materials died out.

As for 'technologies' there is really nothing from the Ancient world that exists that we couldn't produce today; the only mysteries were exactly how some mechanisms worked. For example, Antikythera mechanism was a bit of a mystery at first, but once we worked out what it was for it was pretty clear.

The simplest fact is that most of the 'mysteries' of the ancient world, technology wise, are not really lost or misunderstood, so much as theory remains to be tested. This may surprise you, but not many places are going to authorize funding for large scale archaeological experiments to test theories on pyramid building or to build a full scale quinquereme. So multiple theories on many aspects of ancient technology exist that are yet to be tested on which one is the most likely true in practice.

Are there any medical technologies that have been 'lost'?

Months ago there was Lost skills feature. Someone mentioned certain artisan skills, like specific leather shoe making.

Sorry, we don't allow throughout history questions. These tend to produce threads which are collections of trivia, not the in-depth discussions about a particular topic we're looking for. If you have a specific question about a historical event or period or person, please feel free to re-compose your question and submit it again. Alternatively, questions of this type can be directed to more appropriate subreddits, such as /r/history or /r/askhistory.