I try to open a jpg file with irfanview , but I get the error message :can't read file header. What can I do?

Sounds like a corrupted file. Can you open the jpeg with any other image viewer (including IE or Firefox)?

No I tried everything, it seems the header is not starting with the bytes FF8DFF , so is tehre an easy way to repair?

Try to view it in another program, try windows viewer

You can also try to convert it to another format (worth a try anyway)