WordPress won't work with php5.4

I recently had to upgrade to php5.4 for vBulletin software to run on a subdomain of my site, but now my WordPress (on main domain) won’t work at all! I had hoped the recent update to WP would fix the issue, however it didn’t; so I had to put a plain index.html page up for the interim.

Does anyone know how to fix the php5.4/WP issue? I code but I’m afraid this may be beyond my ability.

Thanks in advance for any assistance.