Computer shut down

Hello, Recently my computer started shutting down all by itself.whether the battery is fully charged or in plug, i switch it on may be watch a movie or listen to a song or read my notes and after 15_20 minutes it shuts down and i have to switch it on again.i have not been playing games with it. its been doing this all the time for 2 days . what could be the problem
and what can i do?