Unable to Retrieve File via Time Machine or External Hard Drive

(1) Open Numbers from the Applications folder and try from the File menu "Open recent" ... the last several files you worked on in Numbers should be listed. If your file is there and can be opened, open it and you can see where the file is located by clicking on the name at the top of the sheet shown when you open it.

(2) To recover a file from Time Machine, first in Finder open the folder in which the file exists/existed. Then select Enter Time Machine from the menu item for Time Machine (looks like a small clock). You can step backwards in time and it should show what was backed up from that folder. You can select a file and restore it from that window.

(3) Not sure the following works if the Time Machine disk is encrypted, but normally you can open the Time Machine disk with Finder (double click on it) and inspect in the Finder each backup set, they are labeled by the date they were completed. You can copy files from there to your Mac.

(4) Going forward, I suggest that you keep two types of backups. The second type could be a "clone" backup, such as is done by CCC or SuperDuper. These can be made bootable sometimes (one can boot from the external drive), but bootable or not, they can be opened in Finder and files can be selected and copied via drag/drop in the Finder. This makes you more immune to some sort of Time Machine mishap, you have a different type of backup to use, just in case. Also good as an independent backup is selective use of cloud storage (Apple, Google, Microsoft, Dropbox, cloud backup services, etc.) where important files can be stored. I recommend, in the case of cloud storage, that you set it up to keep the files also physically on your Mac. Then they also get backed up also by Time Machine, clone backups, etc.

Glad you found your file again!

In case you did not know or remember that folder name, one is supposed to be able to search through Time Machine backups for a file (which of course requires remembering the filename or a part of it, spelled correctly), even if you don't know its containing folder. However that works only if the Time Machine target drive has been connected long enough for Spotlight to index its contents.

Another thing -- I use Dropbox to store certain important files in addition to multiple backups to external drives. I use the option where the file is both in the Dropbox cloud storage and also physically kept on my computer. The importance of this became apparent to me in January when we had a massive wildfire in my city (in Southern California, U.S.) and my entire neighborhood had to be evacuated in the middle of the night. We had to leave the home for more than 10 days before we were allowed back in. I did manage to throw the laptops into the car and all the backup drives but the desktops stayed behind, we had almost no time to pack and leave. Had I been out of town or simply in some other location, and such a wildfire swept through and consumed my home, I would have no record of my files except for what was in Dropbox. I also use iCloud Photos and iCloud Messages, so those are safe as well. The off site storage is also protection against storms, theft, floods, etc.