MacBook Pro shutting down and restarting

Hi,

my MacBook Pro keeps on shutting down and restarting at a random times when using it, when I turn on the Photo Booth, when i want to open an app, when i download something or sometimes even at the sign in bar to access the computer. I went to an apple salon to get it fixed, they changed to a new hardware(computer disk with a memory storage) and the problem went away for one evening but after that it’s been the same as before. Also it often happens when I use word or watching something online and want to open a new tab, often I can’t save my work in time, It’s get Lost after the restart. Please help me, I don’t know what to do anymore :(