What computer operating system is your Premiere Elements 9 running on?
For starters, please go through the Adobe document on damaged (will not open) projects and determine if any of Adobe's suggested troubleshooting there works for you.
At this point, can you open new projects? For that matter, can you open other saved/closed project, but just not this one project? Depending on your reply, an applicable question might be what happens to the issue with and without the antivirus disabled? Lots other questions about computer and/or program updates.
Thanks. Looking forward to your follow up.