I was asked to update new version of nisus express. Did it and now it's frozen, I am not able to open the application "because it may be damaged or incomplete". What happened and what could I do?

It sounds like the download was corrupt, or the update process didn't complete successfully. Either way, it would be best if you downloaded the update manually:

http://nisus.com/update/express.php

Just replace your existing (damaged) application with the new one. Let us know if you have any more troubles.