I am positive I did not delete it, yes. This has happened once before, when I had backups of the Mods folder, but since I do not have access to that external hard drive now, I kind of need to solve this.
When I open the game now it says: "It looks like you have installed an unofficial game modification that does not match your current version of The Sims 3. We recommend that you either uninstall these game modifications or get updated versions of them - otherwise you may experience crashes or significant bugs."
But I don't have any mods installed at all?