My computer turns off by itself!

Hello,

My computer was working perfectly fine until I decided to clean it with an air duster which, I mostly do every 5 to 6 months. I took the majority of the dust out. Not to mention that I took out the heatsink of my processor to clean it and then put it back perfectly fine. I closed the case and plugged all the cables back, turn it on. Then I started navigating the internet doing regular stuff, all of the sudden the CPU turns off, I tried to turn it on right away, it didnt. After like 1 minute, or so, it turns on. I was like okayy, I assumed that It was a bug... And continued with was I was doing and all of the sudden the CPU turns off again. I thought that it may have been the old thermal paste (probably got dried up when, I was cleaning the heatsink with the air duster), so I left it alone and this morning , I went to buy some thermal paste (artic silver 5) I cleaned the old thermal paste and then applied the new one, spreading it around the processor. Everything perfect, I thought the problem was solved, I turn the PC on, everything working fine, so I decided to listen to some music and then watched some video on youtube and then went to watch Netflix all of the sudden, the CPU again turns off by itself, so I tried to turn it on right away and it didn't, after like 1 minute it turned on. I'm stuck people I don't know what to do now...

Can it be the power supply, maybe too much dust went in when I was cleaning it??

My computer is an Amd anthlon 64 bits dual core 3.68ghz kinda old, I have around 4 years with it, bought it brand new. By the way, 2 years ago, I upgraded the video card along with the power supply