i didnt reapply thermal paste.
If you don't have thermal paste on there consider yourself lucky that it didn't fry altogether.
Yeah a PSU can lead to random restarts and even if the PSU is new if it can't supply enough power to the computer or happened to be crappy upon purchase then restarts would be expected.
Programs like motherboard monitor 5 can tell you your temps but you need to know the exact manufacturer and model of the motherboard to set it up correctly.
Use a program like CPU-Z to find out that kind of info.