Pc turns on,then off again,then on repeated

Had my pc abt 3 yrs then the D drive just stopped working (I couldn’t open it,and all the stuff on my desktop was gone) I could still open things like my browser and discord by searching for them in the search at the bottom left,then last night I opened it did some dusting and tried to fix the har drive by checking if it was still plugged in I was going to add more thermal paste then I realised mine was missing (I had already unscrewed the cpu fan and removed it) then it turns on for 2 seconds,then off again back on and off,I then read online to remove the bios battery for ten minutes then put it back in,I did that now it turns on stays on for abt 20 seconds then back off,the back on,I doubt it’s the power supply as it was working A okay the other day