The results for Japan would actually be catastrophic. Global trade (as it exists currently) would largely collapse. This would mean that many materials, components, parts, etc. that they need for manufacturing would no longer be available, leading to an abeyance in production. The glocal economy would collapse, as would many local economies as a result, causing them not to be able to pay workers, etc. This could trigger energy and food crises. Many situations like this would arise as a result of the collapse of Western Civilization.
Due to the instability, Japan could also become embroiled in war itself, suffering because of regional conflicts and power struggles. For example, China would likely be in a much better position after such a collapse than Japan would, due to its size and resources, and it might decide to run military campaigns into Japan, at that point, for any number of reasons. One of which is that many parts of Japan have an excellent climate for food production, especially rice, which, in tough economic times, could feed many people. Another being that, having learned lessons from WWII when China was invaded by the Japanese Empire who occupied nearly 25% of the country (which was a mere 80 or so years ago and is in their recent memory), they may wish to take certain parts or all of Japan to protect themselves, and to project military power forward into the Pacific, the East China Sea (important for maritime trade and access to resources), the Tsugaru Strait (which was important in the Russo-Japanese war), The Sea of Japan (a wateway which connects Russia, Japan, North Korea, and South Korea, etc.).
So, yes, they certainly could lose a lot of their current technology. And without the correct political supports, it might also not be back up and running for a very, very long time either. Worst case scenario: even knowledge of how these things work would be lost.
I recommend reading a few books by Jared Diamond to broaden your mind about this topic, if you are interested. Particularly, Upheaval: Turning Points for Nations in Crisis, Collapse: How Societies Choose to Fail or Succeed, and Guns, Germs, and Steel as well.
Renaldo