You can re-create the original tags and categories hierarchies, but it's a manual task involving 'editing' everyone of the imported tags.

First, try to re-create as much as possible your categories hierarchies. By default, you have only the main categories : people, places etc. Re-create the categories and subcategories you can remember (don't worry if that is only a part of the initial tree).

Then, for each tag in the 'imported keywords' category, right click on the category and choose ' edit keyword'. In the dialog, replace the default (imported...) category by the correct one in the drop down list of existing categories/subcategories.

Edit:

You did the right thing when you saved the metadata to files. Another useful feature of the Organizer is that you can save the categories tree to a text file, and recreate it in a new catalog or add it to an existing catalog.

In the keyword panel, next to the + sign, you have a small triangle : click on it to see a drop-down menu with one option 'Save keyword tags to file'. This will save the tree (hierarchy) either to restore it or to add it to an existing category hierarchy.

If you have to repeat your procedure by reimporting your media in a new catalog, restore the categories before you reimport your media.

I was surprised that there isn't a small file that keeps details on tag names & its hierarchy since there are small file for almost everything else.

You can't find the backup, but you think you will be able to find such a small file (if it existed)?

There is a file with this information, it is the catalog file. You need to develop a better backup strategy so when your computer crashes (actually, when your hard disk crashes), you know where the backup is. In addition to knowing where the backup is, this backup strategy should include regular backups (I suggest weekly, but that's really up to you), and the backup must be on a different drive than the actual photos and catalog.

When I was running PSE, I decided that I was not perfect enough to manually initiate a backup on a weekly basis, I would forget, and that isn't good. So finally I decided to use 3rd party software which could be set to run weekly and backup everything, and/or backup incrementally.