How to Optimize Excel Usage on iPhone X and MacBook Air for iCloud Drive

I have an iPhone X and a 2017 MacBook Air, both with an Excel 365 app. I have an important excel spreadsheet which I have been using for at least 18 months stored in iCloud drive. I regularly edit this file on my Mac. Today I attempted to open and edit it on my phone, which was successful. After doing so, within an hour, I then attempted to open it on my mac and I got an Excel error saying:

Alert - Excel cannot open the file ’___.xlsx’ because the file format or file extension is not valid. Verify that the file has not been corrupted and that the file extension matches the format of the file.

The file extension does match the format of the file and so I can only assume the file has been corrupted. I attempted to convert the file to a .xls file and the resulting document was a one sheet page of strange characters.

Is there anything I can do? This is a really important file and I am desperate to get it back, but have already spent 2 hours trying the different suggestions I have seen online.

[Re-Titled by Moderator]