Mac Pro (early 2008) black screen after 2015-004 Mavericks security update

Hi,

My Mac Pro (early 2008) does not start up after 2015-004 security update last week. I can't

get anything on my monitors. I can hear it starts booting, but not sure if does so fully, because the screen

stays black. Have tried all the different booting options, e.g. safe boot, but no effect. Is there a way to revert

the update or even do a clean OS install, even if I can't see anything on the monitor?

I just noticed that Apple has released a supplemental update today for Yosemite 10.10.3,

"that will fix video driver issuesthat will prevent your mac from starting up when running certain apps that capture video"

I am wondering if this is my problem, even if I use Mavericks. Any thoughts?