Easy Trieve explained. The rest is too much for me to explain, and why I now suspect my hard drive may be playing tricks. I can promise you that the act of inadvertently saving the form caused the only copy of the ODB which I kept in the Documents directory to vanish; my guess is that the ODT file overwrote it for some reason. Same name? Same date stamp? I wish I knew. Thanks to R/S’s coaching I found a copy I had placed some time ago on a separate drive which had not been so altered, and I have
Tip for JG: The way I make backup copies of my .odb files (in Mac OS) is just to select the ‘original’ file .odb icon in the Mac Finder and do a DUPLICATE (command-D). I then change the word COPY in the new filename to the DATE the copy was made such as 2017-03-24. NOTE: The SIZE of the file should more-or-less the same has your ‘original’. (If it isn’t, something is probably wrong.) The technique also works (with minor modifications) to entire SPLIT database folders.
I’m pretty sure LO associates all the files it supports so that LO itself will determine the appropriate path to handle the file on open, instead of associating specific files with specific LO binary.
In any case, OS X (maybe pre-X too ?), Linux and BSD all have file command, run it on the file in question, it’ll tell what it is. You can even get a native one for Window$, I use it sometimes.
(Looks like you pretty much already figured this out, but for others who pass this way.)
What causes this:
Base forms, are actually overlays inside Writer pages. Hence if while in a Form you Save Copy As by default the save will be created as an *.odt. That’s where the trouble begins.
To properly save a copy of the database you need to either close the form, or switch to the outer window, i.e. the window that has a “- LibreOffice Base” suffix at the top of the windows, (not the window with the “- LibreOffice Base Form” prefix).
I fell in this trap early on and had to rebuild to recover. 
BTW, Recent files is no good if the file to open isn’t there to open (like if you moved the file or renamed it). I have to clear this list from time to time after I rename or move files.
(I pesonally don’t know how to recover from this, except to go back to your most recent .odb file. There really should be some sort of warning built in someday.)
This most closely matches my situation: the file is there: it is not useable. If there is, indeed no way to recover from this I agree that there should be some indication for the novice user such as me that I am about to throw away many, many hours of work. There are NO surviving ODB files which open as such. My backup files go all the way to when I first successfully created the database: each and every one opens as an ODT “read only” file, which looks just like my DB “form”, awaiting entries,
So I’m not sure exactly what happened to you. Let’s break this into two, data and program: A1) Do you know which database you are using? Choices: Embedded or not embedded? If not embedded then what? If you don’t know, then it is most likely Embedded. A2) Can you click Menu> View Data Sources? Is your data shown there? Can you copy it out to Calc? B) To not loose changes to your forms and other design, you re-create it manually with the help of View Toolbar Form Design?
One other question. You say has been turned into a text file. But this exactly what do you mean. Do you mean it has extension .txt and you can open it with a simple text editor, like vi, Notepad, or the like? What filename extension is your best, most current file? Does it open with LibreOffice Writer? And if so can you please provide a screen shot?