MacBook suddenly shut down and will not turn back on.

Clean the cooling system, replace the thermal paste, and if you see anything that looks like corrosion or liquid then scrub the board with a toothbrush and 92% alcohol. and if you still have problems you may have to replace the magsafe power adapter and the keyboard with power button. This will be assuming liquid damage is present. Sometimes people are un aware that they have liquid damage. It can happen if a friend spills something on it, liquid is present on the counter, or moisture is somehow present. This will also help with a cooling system issue although in rare times the thermal heatsink has to be replaced.