SOS urgent

Excuse me for the possible errors of this post, but I do not know English and I have translate my request by Google Translate.
Yesterday morning, I was busy writing the rest of a book that I write using Scrivener (version 3.0.3). I stopped work around 1 pm
When I took it back, it was about 2 pm and I never managed to open my file.
After many negative tests, I told myself that I had only one solution: download the demo version to continue working.
That’s what I did.
I have the impression that this is only an interim solution.
How can I go back to the back of my complete and operational software?
Thanks to whoever will tell me.

If I understand correctly, you started this project in Scrivener 2, and decided to try the Scrivener 3 demo, which as recently expired? If you can still load your project, then use the Fichier ▸ Exporter ▸ comme projet Scrivener 2… menu command to save it in the older format.

Otherwise, you can upgrade your copy with a discount, by clicking the “Upgrade from an older version” link, in red text. If you do that you will be sent a new serial number, and will be able to unlock 3.0.3 and continue writing.

Thank you for your answer, but I’m clear.
I have version 3.03 of Scriverner, which I acquired.
In the software preferences, I have this:
"Registered version.
This copy of Scrivener is registered in the name of Guy Gafguy. Thank you for supporting Scrivener. "
But I still have to translate by Google Translate!

Okay, if it is registered then you should have all of the features now. I hope that all is well now?

Yes, everything is fine now.
But I would like to understand the problem I had.
I had to download the demo version hoping it would be enough and apparently that’s what happened;
But that does not explain to me what happened.
Do you have an idea?

It was registered before, and then a day later was not? Maybe something changed with the computer to make it think it was a different one, and so it went back to demo mode. If it happens again you can use your serial number again.

Thanks you.