Can't open display

[stuckat]

I have been using Ubuntu on my laptop since last few months with no problems. Usually I connect to my school server through VPN to work on some text files. Last week I tried to open the a text file on my school server with gedit text editor when system gave the message "Can't open the display". The same command was working until last week and I could open the text file but not now any more.

Any suggestions to view the display