There are two forms of loss, loss of the theory, and loss of the practice. We will lose the practice - all the tribal knowledge that comes from doing it quickly. However so long as we can still read the physics, engineering and math text books we can recreate something similar. It will take a few years and we will make mistakes, but take a bunch of "smart people" and give them time and they could re-create anything from first principals.

It only took a few years for the Manhattan project to create nuclear bombs once physics advanced to the point where we realized such a bomb wouldn't need to be the size of an aircraft carrier. Only a few years after that (and a much smaller budget) to make a non-exploding power reactor. There is no reason society couldn't duplicate that if we wanted.

Now the tribal knowledge does help a lot, it you still have it you can take a lot of time off. However it isn't needed.