While I was upgrading to Ubuntu 15 (the development version), my system crashed. After I restarted the laptop, I saw a kernel panic. I also did a memtestx86 which didn't find any error.
more info :
Before doing upgrade i have changed my default desktop to gnome which by entering wrong commands i lost some packages .to revert the previous desktop i removed all the packages and then installed them then i face that some packages weren't still installed . after i run terminal and started to upgrade ( i found it good to troubleshot the problem by upgrading somehow ) however after crash i got that screen by the way i have faced another screen after this one which backed to the first again