Talking [ask] all setting back to first install after 1 week

hi

yesterday i have problem with my nexus 5

firts of all, i got carbon rom + elemetalx kernel

everything was ok until after 1 week suddenly my screen got many blank screen or black screen but if i push volume it had sound volume up or down. i decided to restart, but the problem still occur, until the next day my nexus 5 return to original state (like first install rom) the app still there though and return with play service fc.

i wonder what causing it,

fyi i reset my rom and everything ok again but still curious what happen

thx