I started a new Unity Project and then i made a terrain (3D object of Unity). After that I wanted to Import a fbx-model in the assets Folder, but before I could do that, Unity crashed and I don’t know why.
Unity crashes for myriad reasons. All you can do is file a bug report when the crash window shows up. Or go to Help > Report a Bug...
, then pray to whatever god you think will influence the developers to solve the issue. It’s really all you can do I’m afraid.