1 - You have a backup - keep it saved and if you need to use it first make a copy.
2 - Make a copy of the file with problems without opening the file - use copy and paste according to your OS.
3 - Open the Application folder of LibreOffice - you find the path to the backup folder: Tools > options > LibreOffice > path
4 - Look if you find there a copy of the file which creates problems; there is a chance to find also a former version which can obtain the information you are looking for
5 - Save all the files from the back-up folder into a safe place by using only copy&paste according to your OS
=> now you have safety copies of all files which can be important to you.
6 - Make working copies of all saved old files - again by using only copy&paste according to your OS
7 - Try to open all files with LibO Calc and see which file is the best for you.
8 - If the important file only opens as text import - the gibberish and symbols you can avoid by choosing the right character set. This depends on your language. I would start with UTF-8 but in your case other character sets can fit better.
And additional comment:
Very often I see questions on how to recover lost files. Better than attempting to recover lost data is prepare against the loss of data. To avoid a loss of a file these are 3 recommendations:
1 - Check the following LibO options: Tools > options > Load/Save > General
---- Save Autorecovery every x mins (the shorter the value the small a possible loss; I use 3 min)
---- Automatically save document too ( I don’t use this, because I regard the autorecovery function as excellent.)
---- Always create a backup copy (download and read the free of charge LibO guide to understand how this works)
2 - When I work more than a day on a file, I create additional daily back ups. I use the format “filename_yymmdd.ext” and I keep at least 1 previous version.
In case the file is extremely important and I work long hours non-stop, I add the actual time behind the date.
3 - Backup the HDD of your computer to an external HDD. External HDD in the range of 2-4 TB cost mostly less than 100 EUR. (I do this almost daily and even backup external HDD onto another one.
When it comes to this topic this expression comes to my mind: “Fail to prepare is prepare to fail!”
