Why does It keep rebooting itself?

Hi!

My Chromebook at first was rebooting itself. It would restart itself while I was doing my work for college. It started doing it once everyday but then it got worse. I was at school one day and it was charging and it completely shut off causing me not being able to hand in some work. I really need to get it fixed but no idea whats wrong with it. I could only keep one tab open or it would reset