If you're in the middle of a project, you should roll back to 4.1 and finish it. You should not upgrade FCP while working on a project with a deadline -- never do that again!
I'll assume you've already completely shut down your system at least once since upgrading. If not, do so.
FCP might be crashing as it tries to load your project -- worse, it might corrupt your project as it tries this, so rename or move your FCP project file to keep FCP from auto-loading it. Does FCP load now?
You'd better call Apple and pay for some tech support, if that project's important!