Excel cannot open file due to invalid file format or extention. File name has been changed to .~lock

Hi there! Would love some help with this one.

I was working on this worksheet when my pc crashed and has to restart. I click the file again and excel crashed.

Then, then I restarted, file name was changed to .~lock.Tomt testskjema ny.xlsx# (new additions to the file name in bold).

I changed the file name to its original name; 'Tomt testskjema ny.xlsx'. I then get the 'Excel cannot open the file <filename> because the file format or file extension is not valid. Verify that the file has not been corrupted and that the file extention matches the format of the file.

I have tried change file extension and 'open and repair' feature, with no luck.

Any tips would be greatly appreciated.

Could not attach the file, due to the below error:

• The attachment's tomt testskjema ny.xls.xlsx content type (application/vnd.openxmlformats-officedocument.spreadsheetml.sheet) does not match its file extension and has been removed.