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Sorry - your file is full of zeros or null characters ...
If you are lucky, you might be able to un-delete some of the temporary files which Writer wrote while you were editing the .odt file, and then deleted when you closed Writer. Act quickly - they risk being overwritten the longer you leave it.
See
[Tutorial] How to find and un-delete Writer temporary files for
a) detailed instructions on how to recover your file as it was when you last opened or saved it, or as it was when it was last saved with AutoRecovery.
b) how to find previous versions of the file in the folder it is located in, but which have since been deleted.
c) how to un-delete the temporary files Writer wrote while you were editing the file, and then deleted. This will recover your file as it was when you last opened or you last saved it and is probably your best hope.
Edit: Edited to point to the Tutorial |