Having the system locked on a black screen or the boot log screen is a common issue. It's almost everytime related to gfx drivers. To be able to reach the GUI, follow those steps, according to your hardware.

ATI :
QUOTE
Boot and get the boot prompt with the F8 key, type -s and enter.
This will get you to the single user mode.

Type the following :
mount -uw /
rm -R /System/Library/Extensions/ATI*.kext
reboot


nVidia :
QUOTE
Boot and get the boot prompt with the F8 key, type -s and enter.
This will get you to the single user mode.

Type the following :
mount -uw /
rm -R /System/Library/Extensions/NVDA*.kext
reboot


Intel GMA :
QUOTE
Boot and get the boot prompt with the F8 key, type -s and enter.
This will get you to the single user mode.

Type the following :
mount -uw /
rm -R /System/Library/Extensions/AppleIntelGMA*
rm -R /System/Library/Extensions/AppleIntelIntegratedFramebuffer.kext
reboot

Your system should now be functionnal again, however it's using VESA mode (generic driver without acceleration).
You now have to set your graphic driver correctly.

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In case you'd want to restore the stock kexts :
QUOTE
Get Pacifist
With it, open /System/Installation/Packages/OSInstall.mpkg (should be a hidden folder) on your DVD.

Now restore all the files you removed earlier : everything in /System/Library/Extensions/ starting by either (following your case) NVDA, ATI, or AppleIntelGMA & the AppleIntelIntegratedFramebuffer.kext
Then from the COMBO update that matches your current version, do the same over the files you just restored, and say that you want to update when pacifist asks.
*In some cases you'll also want to restore /System/Library/frameworks/OpenGL.framework. This folder must match the gfx kexts version.

Updating the kexts with the combo update is not allways mandatory, however it's highly recommended to avoid problems of version mismatches.


If the stock kexts also brick your installation, maybe they require to be modified before you reboot, or your hardware is just not supported.

Now it's up to you to find how to proceed in the forum.