Why does my computer keep shutting down when starting a game?

Question

Iron

A week ago, for the 1st time, my computer was cut off when playing DotA, and now it is cut off every time I start it, and I would like to know what is the reason.
I have 2 hypotheses:

• The 450W power supply does not withstand (although before that everything was fine for 2-3 months)
  
• Either the CPU or something else is overheating
  

Answer 1

BP.
Clean it from dust, change capacitors, buy a new one for 500 :)
PS about CPU overheating, it does not overheat instantly, if you have a temperature of 30-40 in idle or minimal loads (surfing, etc.), then you can not sin on cooling.