It was my RAM problem which is now solved but I just got this error two times and system was shutdown automatically but now its running fine.

Code:

This system is shutting down. Please save all work in progress and log off. any unsaved changes will be lost. This shutdown was initiated by NT Authority/system.

MESSAGE
the system process 'C:\windows\system32\lsass.exe' terminated unexpectedly with status code -1073741819 the system will now shutdown and restart.

What was the problem ?

Most probably sesser worm problem. If your computer is now working fine then first thing you should do is update your antivirus and run a full system scan. If still you face same problem, let me know here.