Citavi 6.5 only
You added a Word document to a reference in your project while the preview pane in Citavi was enabled. Citavi then froze.
Solve the immediate problem by closing any other Word documents that are open. Then, use the Windows Task Manager to end the running Word process, which is responsible for the preview of the Word file in Citavi.
As a workaround, avoid the problem in the future by switching from the preview pane to the quick help before attaching a Word file to a reference.
This problem will be fixed in the next Citavi update.