Hello everybody,

I completed The Witcher 3 a long time ago but I still install it every now and then out of nostalgia and to replay some parts of it. But the last time I uninstalled it from my computer, it just disappeared from my account, eventhough I can still see my hours played and achivements. Do you know anything about this and/or how can I fix it. Thank you in advance for your help.

If I understand your post correctly, you are using GOG Galaxy.
Please, check if the game is still in your library on GOG website: if it's not, you have to contact support.

Go to your account, search for witcher . If there's a box at the bottom saying 'Show hidden games' check it.
If the game is hidden, this should make it visible. You can than unhide the game in the options.

If that doesn't help, the game was probably removed from your account due to a fraudulent purchase. You might have gotten the game in a trade or from a shady key side like Kinguin or G2A.
You can contact GOG and see if they can clear things up. However if the code originated from a 3rd party, you'll eventually have to contact them.