Depending on the size of your original data file, the answer might be to create a new excel file from an original data or data source.
Failing that, reconstruct a new excel file from the data within the QGIS layers attributes -- that is, so much as is available within QGIS to go back to excel
Failing that (and this is where I suspect you would end up...)...
[MAKE COPIES of ANY FILES you are playing with, store the copies in a separate folder and EDIT ONLY THOSE COPIES in that separate folder !!!]
Append the extension ".zip" to your crashed xls file -- later, you will go exploring inside the zip file. You'll discover a bizarre collection of folders and files, but they will contain, among them, your data in XML format (on assumption your original data is stored in a recent excel format, to begin with...)
Create, as part of that exploration, a new excel file, with the data headings you expect in your original excel file, but with one line of fake data. Do NOT amend or change the default formatting in this file. On the assumption that your original, crashed file used one sheet, delete the second and third default sheets in this new file. Then, after saving the file, rename it with the zip extension, and go exploring, there, to see how your data is laid out in the XML format of that one-data XLS file. Because you did not add any formatting, you will be looking at a minimalist version of the file behind the XLS/Zip. This will help, in the next steps.
Then, go back to your crashed file, with its added ".zip" extension, and look for your data. Cut and paste what you can, back into your one-line data file, taking care to be aware of proper XML formatting. You might have corrupted data or files, or characters, or maybe missing some XML, or other problems in your crashed XML file.
For the exploration of the XML files you find, use "Notepad++", as it will highlight the XML formatting, making your discovery process easier.
All this, because I take it your data is very valuable, worth the time to recover, and you're willing to go on this experiment trail.