A "white screen"? You mean the whole screen was white?
If not, did you try doing what the message tells you?
Hint: actually read the message in the dialog :p
Yep, the whole screen was white, apart from the exit button.
If you have access to the menu bar, click on 'Controls' and select 'Show Debugger'; then in the lower right of the debugger, click on 'Terminate'. Maybe I am mistaken, as I do not see an Exit button in my version of Greenfoot.
I don't have access to the menu bar, and by the exit button I mean the cross in the Ubuntu toolbar at the top of the screen. I'm having trouble finding this debug log text file though, where would it be located?
The debug log file can be found in:
~/.greenfoot/greenfoot-debuglog.txt
Where '~' is your home directory.
Found it now! Thankyou so much:) Don't know about you, but I hate hidden folders with a vengance:)