Excel file format or file extension is not valid

I hope one of the experts here will be able to save the files, which are very necessary for my work. My excel file became corrupt for very obvious reason..my little brother just deleted it it was no intentionally, but i`m still disappointed. I have tried to recover it with some program. The file appeared - everything is ok, at first sight. But it doesn`t want to open. I get an error message: Excel cannot open the file because the file format or file extension is not valid...File is corrupt.
So, maybe anyone know how to solve this problem. I will be very appreciate for any advice.

P.S. I use MS Office 2007 with Windows XP

How did the file surface? where was it located?

have you tried Open and Repair command? If not, then try it. Sometimes it can recover the file.

And make a back-up and reinstall Office.

In this case if it doesn`t work, you should use other methods for recovery. Probably, your excel file is seriously corrupted and you need third party solution to get it back. I can recommend you Recovery Toolbox for Excel https://excel.recoverytoolbox.com/
You also may try some powerful methods like checking your backup files or make system restore.

Could you attach any copies of the file that you have recovered so we can examine it?