After System Update last night, the system will not boot with IOPlatformExpert.cpp:1504 error
Tried clearing your PRAM? Hold down Option + Command + P + R before turning the mac on, keep holding them until the mac makes the startup chime again (twice in total).
Apart from that, boot off the recovery partition and reinstall over the top of your hdd. Your data won't be changed, the OS will just be repaired.