PC wont turns on&freeze after playing game

Hey guys, my pc is around 5 year+ old. After playing a game, the next day i try on my pc, its become on and off, and i have to switch off. Sometime my pc became freeze while watch movie online and playing game.
But currently now, my pc working well and smooth, but i didn't use it to play game for a while. So guys, what do you think the problem?
Should i upgrade some of the hardware and which should i upgrade first?

This is pc spec:

Window 10(10.0) 64bit
Intel® Core™ i5-2300 @ 2.80GHz
Gigabyte H67MA-UD2H
8GB DDR3
1TB HDD
NVIDIA GTX660 - (Plan to change to GTX 1050TI 4GB)

Need you guys advise - planning to upgrade this June18(after save some money)