That sounds incredibly frightening and am sorry you had to go through that. That would certainly be enough to convince many people not to ever fly again.
Turbulence not related to convective (thunderstorm) activity and in clear air is simply referred to as "clear air turbulence" (CAT) and it can often be much more severe. Worst yet, unlike turbulence within clouds, it is unable to be seen by pilots and thus harder to avoid. The best tool we have at avoiding such turbulence is by reports from other airplanes, known as pilot reports, or PIREPS. Certain types of CAT is known to be more likely with certain conditions. One of these is known as a mountain wave. Mountain wave turbulence can be very severe. Based on your location, and depending on the winds that day, this could have very likely been the culprit. It wouldn't be surprising if a preceding aircraft flew that airspace at a different altitude and reported moderate turbulence to air traffic control, and your flight flew the same airspace at a lower altitude that was much worse.
Of course, this is all conjecture. As to the engines both failing, that part I can't help you. That seems highly unlikely, but stranger things have happened, and I am not familiar with all of the systems of a 737-300. Perhaps the generators came off line. No idea.
Glad you're thinking about flying again, and know that this type of thing while very scary is also very rare. Further, even if it does occur, like you saw, it's not going to bring the airplane down.
Read about mountain wave turbulence here:
https://www.weather.gov/media/public...1nov-front.pdf
and here
https://disciplesofflight.com/mounta...t-flight-ever/