I was using gentoo with kde recently, and it was looking fine after booting.

But when I started the konsole, the screen will be black suddenly and the keyboard & mouse did not work. The power was still on. I had to press the powerbutton to restart the computer.

that above on had happened many times. I have no idea about it.How would you tell me the reason and solution?

Thank you for your help.

Just to clear it out, when you say "konsole" you area talking about the KDE terminal emulator, right?

If so, it might be a bug in either kde or your graphics driver. I haven't had much luck with kde 4.x, using either nvidia or ati cards I've only found problems everywhere, specially when dealing with dual head setups. I can't tell you how to solve your problem, however I advice you to try an alternate window manager like fluxbox or whatever you prefer and see if you can reproduce the problem there.

It could also be a hardware problem.

Strange problem, but I've had good luck so far with the latest Nvidia driver 195.x.x and KDE 4.3.5. You didn't mention what kind of video card you have and what driver you're using...

You can try just restarting X with ctrl-alt-backspace to save having to reboot every time (you have to setup /etc/hal/fdi/policy/10-x11-input.fdi as described in the manual for this to work).

You could also try ctrl-alt-f6 to get to another terminal from KDE when the black screen occurs. Take a look at the system log files and see if anything looks out of place (dmesg, /var/log/syslog, /var/log/Xorg.0.log, etc.).