[Pharo-project] pharo system

[gorgui ndong]

Hi,
I'm a beginner in pharo, and sometimes the pharo system is completely down. It 
don't give me a feedback for say me why this system error occurs. So I want to 
know what are the main causes of pharo's crashes and if i'm in this case what 
can i do for not loose my last image, with my last changes.
Thanks 
Gorgui