Error "misc_appliciation.ai" is in an unknown format and cannot be opened"

Hi

Recently, I tried to create a file in ai, and saved under the name "misc_appliciation.ai". I do not have any problem previously trying to open it, however, today, for resons unknown to me I was prompted error "The file "misc_appliciation.ai" is in an unknown format and cannot be opened" when i tried to open the file. Kindly advise why it would happen since the file is in .ai format.


Thank you.