After security update, macbook pro won't boot

5-month old mb pro running OSX Lion. I ran last week's sw update, now it won't boot up. I got into disk utils, checked the drive, it's fine. I booted from an external usb drive, and see that everything is still there on my original drive. How do I fix this? I can't just restore, bc I put a lot on it since my last time machine backup. If I install the OS fresh, won't it clobber all my installed sw and configs?

Thanks,
V

G'day and welcome to the forums.

Not if you simply do an install without formatting the drive. All your files etc will be intact however depending on original install may well take it back to OS X.7 and you will need to update again.