I have ubuntu 9.04,but whenever I start watching a movie my laptop hangs and does not respond , I need to restart it work.I cannot understand the problem ,it has happened thrice now.
Please help.

A few more details would be helpful.
1. What kind of file where you trying to play?
2. What player were you using?
3. What error messages (if any) appeared?
4. What video card do you have?
5. What driver are you using for that card?
cheers,
jdk

the problem occurs when I click on "view full screen".It just does not respond.It gives an error message which is just flashed and I am unable to read it.The file I am playing is an .avi file.
01:00.0 VGA compatible controller: ATI Technologies Inc M92 [Mobility Radeon HD 4500 Series]
it is ATI Radeon HD 4570 card

Can you please please please tell us what "it" means (not the error message "it"; the other "it" that is not responding). In other word what player are you using? If you run whatever the player it is from a command line then you might be able to see the error messages properly.
jdk

It sounds to me that it does go to full screen but you can't get out of the full screen mode. Try [CTRL][ALT][F2] to get to the virtual terminal and use the "killall <playercommand>" command to kill the player program. You could do as suggested, starting the player from the terminal, but also redirect stdout and stderr to a file. Then you can get to the log later.

For example:
mplayer -fs MyNewPuppy.avi 1>puppyvid.log 2>&1

My guess is that if you could get out of the full screen mode, that you would be able to quit the player. The computer might not be locked up, you are just locked out of the regular display by the player.

I installed vlc and the problem persists.
The problem is that while watching a movie when I click on "view full screen" ,the pc hangs ,i.e, a list of error messages is flashed for a second ,some of its content is
Starting System Tools
Checking battery state
/dev/sda:
Checking Power-management
and others.........(which I could not see because it is just flashed for a second)
after this screen is flashed ,the computer visual gives a black screen and nothing works not even (Alt-Ctrl-F2).

maybe running vlc from a terminal will allow you to see the messages although I don't know how because you're running fullscreen and can't clt-alt-f2 to a virtual terminal.

Are there any error messages in /var/log/Xorg.0.log or ~/.xsession-errors that are useful?
jdk

Try using scaling instead and scale the video to the size of the screen.

Pardon me for my ignorance but how do we look for errors related to this problem in ~/.xsession-errors or /var/log/Xorg.0.log
Please help!

In a terminal type the following:
Hit space bar to read more and type q to quite. Do the same command but with the other file mentioned to read the 2nd file.
cheers,
jdk

A little off topic. But hey jdk, whats that penguin head in your profile?

Hi Chaitanya,
I would guess it marks your distro. So you have a Buntu one and I guess my penguin head is for debian. I think it's a feature they added to the LQ site recently.
cheers,
jdk

Thanks. And I guess I should stop posting this irrelevant things here now.
PS: I did not have that logo in my profile thats why was just curious to know about it.