i hav ubuntu on my system...i had made some changes in the login screen and the next time i restarted i cannot get the login screen,i get a blank screen with a 'wait' cursor...i can login using the non GUI (alt-F1..F7) login windows and acces all files etc but cannot get a GUI....need urgent help...

Debugging step-by-step.. Does run X? Does bring X up? Does renaming .Xsession help?

thanx 4 replying...wen i logged in as root and entered 'startx'
i got a lot of errors(fatal)...ther was a message to remove /tmp/.X0-lock and start again...i did it n entered startx again...it partially worked as to i got the GUI...(not the sam desktop as i had though!!!)i tried to reverse wadever i had changed the other day...but it din help...now i hav to do this on every restart and the system feels a bit slow...can i do something (like reinstalling some packages) to get evrything right back again???btw...if it helps ther was som eror on the CLI about missing /dev/input/wacom

Well, /dev/input/wacom should be device for Wacom pen pad (or how is it called), that is mentioned "just in case" in default configuration and can be ignored. Now, if you boot, then remove .X0-lock, then reboot, then run 'init 5', what do you get?