I got a software update on my Snowleopard 2011 MBP today. So I did the install and, at reboot, I get the grey bar and spinning wheel. It installs so far, then the grey bar starts again. Then it gets so far, and then the machine switches itself off. Each time I switch it on, the same thing happens. I think the update mentioned something about the Thunderbolt port. Now I can't boot. Help please!