Hard to guess.
You pulled the 2nd video card. Helps to list what it was, power it needs, and what your other gpu is.
hard to believe you use a 2009 and only 10.6.2.
pull the hard drive you boot from. Got a backup you can boot from? Those are so useful and easy they are musts.
Boot from another hard drive or use Recovery Mode (lion or above) or try Safe Boot - but after you undo the changes.
And hit power button - with all cables and power unplugged - for 10 seconds, then reconnect the kb/mouse and power - that will reset the SMC.
Install OS X to another drive if you still can't boot.
GPUs went. The Apple 5770 required 10.6.5 and up.
I agree that being on OS X 10.6.2 doesn't make any sense. If you're staying on Snow Leopard because of software dependencies (I am) then at least upgrade to 10.6.8 and get in all the security patches, etc. to protect your system fully.
If one of your video boards is a 5770, it alone can run three monitors but as The Hatter says, it requires at least 10.6.5.
Thanks both - not sure how the 10.6.2 crept into the discussion - my mpro was running mavericks at the time - I also have snow leo on another hard disk.
The card normally in use (5770) has been troublesome with 3 monitors since I installed mavericks which is why I thought to add an older card to run a single apple monitor from with the other 2 (a 30" spectraview 3090 & a another small apple monitor).
I have restarted in single user mode and run fsck which claimed to have done some repairs but the problem persists, I'll swap the mav hd out and try with snow leo.
10.6.2 "crept in' because it still shows on hardware profile line and it is not supported, there were bugs in the 10.5.7 thru 10.6.2 dealing with processors over-working and heat. Basically we need more details than just the symptoms.
Did you ask and try to get help with the arrangement of and what active adapters you are using to get 3 displays working properly?
With the 5770 you could use a GT120 or another 5770.
Apple Mini DisplayPort adapters: Frequently asked questions (FAQ)Guide to Apple graphic display adapters needed
Atlona (Dual Link) DVI to Mini DisplayPort Converter for Apple 27-inch LED and 27-inch iMac
Mac Pro (Early 2009), Mac Pro (Mid 2010), Mac Pro (Mid 2012): Issues with three displays and multiple DVI, HDMI connections
It's not the USB data that is needed to run three displays. It's the extra POWER. If you read the Apple article, they say the only solution they endorse is their US$100 Dual-Link adapters. Check the table at the end of the article.
Readers here have found that if you have two Brand-X Powered adapters, aka active adapters, it also works fine. These do not always require another USB. If designed recently, they can pick up a power pin in the Mini DisplayPort interface.
AMD (makers of the 5770 and 5870) lists the qualified POWERED adapters that meet their most stringent requirements. Look down the page and you will find several qualified Active Mini DisplyPort to various others, such as HDMI. These adapters are available from other outlets, such as Amazon, sometimes for far less than the Apple adapters.
The other desktop monitor will plug directly into the DVI port on the card and the 40 inch tv will still connect via HDMI over the second Mini Display port.
That will work only if you use an active Mini DisplayPort to HDMI adapter.
The only (someone) have found to work (2009)