Installer Issues via Installer -- Not for global upgrades or upgrading individual packages -- ONLY ISSUES WITH INSTALLATION OF THE OS -- Can't get Sabayon installed, than post here, otherwise use correct forums
This could be video driver related or login manager related. Is it after you login to kde or once kde starts to load / first graphics part or during kde login and the loading icon thingy's is on the screen ?
You should be able to press ctrl+alt+backspace and this would reload the x server / kde part. Then at the bottom is a menu for console login and then login as root into text looking place.
Next run dmesg and the last few lines should give you a idea of what is going on.
Or at hd boot time press e to edit the grub boot , might have to press it 2 times so you can edit the lines there and add nox then press b to boot or twice pressing b to boot and once in text login as root you can start x server with startdm or startgdm not sure here , but this should get you into kde as root user + then you can edit /etc/X11/xorg.conf if needed. If this last way works then the login manager is probably busted / corrupted and can be fixed.
You can also try the wiki on top of this page / gentoo.org forums / wiki as well.