Excel error, canīt open file.

Good afternoon.
How to solve this error?
I think I've tried everything and I can't open the file.
Thanks.

The error:

"Excel cannot open the file .... because the file format or extension is not valid. Check that the file is undamaged and that the file extension matches"

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The error message tells you that there is something wrong with either the format or the extension used for the file in question. We cannot tell you any more than that - sorry.

It looks like the file in question is corrupted. Find and use a backup.

What can i do besides that?
I don't hav any backup!!

I don't think there's a lot you can do. Attach the file here and one of us will see if we can open it, but if it is corrupt and you have no backup (not good - you need to sort out a robust back-up regime pronto!), then you will be starting from scratch, I am afraid.