I've just spent hours writing a very important document, constantly saving so that I would not lose my work in the event of a computer crash.
Of course, my computer just crashed. So I restarted it, and went to open the document.
The icon has changed, and instead of showing the usual icon for .odt files, it is different. This picture shows the difference between the file I was working on (on the left) and the other file I had open at the time, which I was using to copy-paste some passages across. All afternoon, while I was working on it, and saving the file at regular intervals, both files had the same icon.

When I tried to open the file, I was presented with a box telling me to register OpenOffice. This is bizarre, as I've always used OpenOffice on this computer and have never seen that before. Then, I was presented with a box of ASCII Filter Options, which I've also never seen before.
Next, I get this box, which I've also never seen before. Needless to say, I have not locked the file:

On opening the document, it is completely corrupted - all my work has gone, and has been replaced by hundreds of hashtag-type symbols.

The other document that was open at the time - the unimportant one that I was just using to copy-paste stuff - is completely fine.
I have no idea what to do. I've Googled for tutorials on how to repair corrupted files, and they either don't work or I can't understand them. Please could someone explain to me what I need to do to get my work back?
It's absolutely ESSENTIAL that I recover all this work AS SOON AS POSSIBLE. There is no possible way for me to repeat the work that was done, and I don't have time left anyway. It's absolutely vital that I get all this work back, and very very soon. Please help. I have been using OpenOffice for more than a decade and nothing like this has ever happened before. Please, please save me...