Welcome, Jafudà! 
What do you mean exactly by " impossible to access to the contents"?
Because you say you can go to the upgrade manager, so you have access at least to some content.
Did you try using the safe mode?
Do you have a backup of your forum?
When things go wrong, sometimes a rebuild of your forum can fix it.
To do this, log in to your server with SSH, go into your Discourse directory
Then, run ./launcher rebuild app