I can understand importing something might break a project but not any project you open. The standard first thing to try for a specific project is to delete its CPD file and then reopen the project.
Are you using dual monitors? If so, try holding down the Windows key and pressing the left arrow. Maybe some dialog is "appearing" on a second monitor that isn[t connected, if you see what I mean.
See www.grainge.org for RoboHelp and Authoring tips
I like Peter's idea about using the Windows logo key + Left arrow. When you try that, not that you should tap the Left arrow more than once. Needs to be at least two presses.
None of the options worked. Reinstalling RoboHelp 10.0 did it!