Does this problem occur with all projects or just one? If you only have one project, create a new one just to test with a few topics or open on of the supplied samples.
I suspect other projects will be OK and if that is the case, then rename the CPD file in the problem project (via Windows Explorer) and then reopen it. RoboHelp will create a new file and all should be OK. Post back if not.
There is a full explanation in Opening Projects on my site.
BTW. How do you click the Enter key? I have to press mine.
See www.grainge.org for RoboHelp and Authoring tips