Playing video, screen become black
 

I have problem when playing video. Sometimes the screen becomes black and I can't do anything when it's happened. Can anyone help ? I'm using ubuntu 8.04

Can you open the video player in a terminal window and see if it puts out any errors? If so post the errors, this is not my strong suit but I'm sure any errors will help figure out the problem. It may just be lacking some codec, (just a guess). Give some system spec also, and what video player you are using. Info always helps (we can't read minds).

What is your video card? How much memory is in your system? Do you still have sound when this happens? If you hit alt-tab do you see any windows? How about ctrl-tab? If you hit ctrl-F1 are you taken to a console window? (ctrl-F7 to return to X).

It's no effect pressing alt + tab. Just blank and can't do anything. And the computer still on.

I use ubuntu 8.04, but this problem isn't happened when I use older version of ubuntu

Well, you didn't answer the rest of the questions, and I consider it a waste of my time to ask twice.