Mac pro mid 2010 - screen suddenly turns black

Try resetting the SMC and NVRAM

Intel-based Macs: Resetting the System Management Controller (SMC)

About NVRAM and PRAM

Try booting in Safe Mode

OS X: What is Safe Boot, Safe Mode?

Try booting from install DVD and see screen works OK.

If you can try repairing the startup disk via Disk Utility.

Run Apple Hardware Test

Using Apple Hardware Test

This is the Mac Pro desktop forum. You do have a 40 lb silver Mac Pro tower?

What about Safe Mode?

Starting up in Safe Mode

Follow these steps to start up into Safe Mode.

1. Be sure your Mac is shut down.
2. Press the power button.
3. Immediately after you hear the startup sound, press and hold the Shift key. The Shift key should be pressed as soon as possible after startup, but not before the startup sound.
4. Release the Shift key when you see the Apple logo appear on the screen.

After the Apple logo appears, it may take longer than usual to reach the login screen. This is because your computer is performing a directory check as part of Safe Mode.

To leave Safe Mode, restart your computer without pressing any keys during startup.

Do you have a backup of all your data?

If so boot from the SL install disk and and format the disk and reinstall the OS.

Otherwise I would boot from the SL install disk and install SL on another disk and boot from that and see if you can get your data fro the problem disk.

I would try a fresh install of SL on another disk. If that works then it is a software problem. If it does not work then hardware problem. likely a graphics card problem. You can try reseating the graphics card

Safe Mode does the equivalent of a Disk Utility (Repair Disk) while it puts up that progress bar. It never get more than about 1/3 of the way across, even when everything is great. But if your drive is un-mountable after the repair cycle, your Mac cannot start up, and it will shut down.

NOT shutting down is a good sign.

Safe Mode should continue and load a minimal set of Apple-only Drivers. It does NOT load the graphics card Driver, which is why it can be useful to learn more about your graphics card.

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If you are using your Boot SSD for everything, and are not using TRIM, your SSD may be clogging up with deleted data. There are several ways to eliminate or reduce the deleted data.

One way is to run any system, 10.6.8 or later, with TRIM Enabler installed, and run one pass of Disk Utility (Repair Disk) with TRIM active. This presents one additional line of output:

"Trimming unused blocks"

and pauses for up to a minute while the unused blocks are TRIMmed.

TRIM Enabler has become much more difficult to install on 10.9.5 and later, because it conflicts with the new Kernel-extension signing feature.

Edit> If you are really running 10.6.8, you can install TRIM Enabler in your active System. It is free, but be sure to get the correct version for your system. There is an advanced-features version also available for a modest charge.