This is not a simple thing to fix.  I'm assuming that you're unable to use the GUI to change back.  You can certainly edit you xorg.conf file directly.  One way is from the command line, using:sudo dpkg-reconfigure xserver-xorgThis will run you through an xorg setup and you should be able to select the resolution you had before you changed it.If you want to add additional resolutions (like 1024x780), you'll probably need to directly edit the /etc/X11/xorg.conf file.  Where you see the lines for resolutions, add the one you want.CAUTION: Playing around with xorg.conf directly can have serious consequences.  The file controls nearly everything you need in the GUI interface (mouse, keyboard, screen).  So don't do things casually.  MORE IMPORTANT: MAKE A BACKUP OF YOUR XORG.CONF file before you change it.

I'm new to linux Can you expalin it little bit PLEASE

First get into a terminal or konsole. Change to the directory you need to be in by entering:

cd /etc/X11/

Now, to make a backup of your xorg.conf file, enter:

sudo cp xorg.conf xorg.conf.bak

Now, enter the command:

sudo dpkg-reconfigure xserver-xorg

A program will run that will take you through the process of setting up xorg. In 99% of the cases, you should accept the default. But in the place where resolutions are requested, you'll be able to get the resolution settings you wanted.

If for some reason it doesn't work, then you can get into a terminal or konsole and restore your backup file:

cd /etc/X11
cp xorg.conf.bak xorg.conf

I m new to linuw and my ENglish soo can you explain me little bit i cant get the things that i should type in terminal soo please help me THANKZZ

Type into the terminal:

cd /etc/X11

Then type into the terminal:

sudo cp xorg.conf xorg.conf.bak

Then type into the terminal:

sudo dpkg-reconfigure xserver-xorg

Then follow the instructions in the program that shows there.

I did it.But not sure that I did them CORRECTLY.I tried 2 TIMES.It s tooooo COMPLEX for ME.(IT'S MY FAULT)So PLEASE CAN YOU GIVE ME EASY WAY TO SETTLE THIS.any oneTHANKZZZZZZZZZZ

Try this:

In the terminal, enter:

cd /etc/X11

Now enter:

For Kubuntu:
sudo kwrite xorg.conf

For Ubuntu:

sudo gedit xorg.conf

In the file that opens, you'll find a section that looks like this:

Section "Screen"
Identifier "Default Screen"
Device "Generic Video Card"
Monitor "Generic Monitor"
DefaultDepth 24
SubSection "Display"
Depth 1
Modes "1280x800" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 4
Modes "1280x800" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 8
Modes "1280x800" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 15
Modes "1280x800" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 16
Modes "1280x800" "1024x768" "800x600" "640x480"
EndSubSection
SubSection "Display"
Depth 24
Modes "1280x800" "1024x768" "800x600" "640x480"
EndSubSection
EndSection

On every line that starts with Modes, add (in quotes) the resolution you want as your best one (in this one I'm showing you, it's "1280x800")

Make sure that the resolution numbers look EXACTLY like the ones in your file already, adding only the numbers you want. And they should be in order in the line, from highest resolution to lowest.

Then save the file. You'll need to re-start X in order for the change to take effect. Then you should be able to go to Desktop settings and re-set the resolution to the one you want (which should now be there).

REMEMBER: If it doesn't work, you'll need to copy the original xorg.conf file back to restore your system:

sudo cp /etc/X11/xorg.conf.bak /etc/X11/xorg.conf

thank you very much randcoop" in fact i have not yisit internet for while. Glad to tell that your first solution worked though i said didn't. I logout and login nothing happen. But after restart it has worked. I m realy sorry for my stupid reply. I like to check this way too. Now you have helped me twice "randcoop". So thank you very much. KEEP UP YOUR GOOD WORK. THANK FOR YOUR CONCERN............................