CPU overheating at around 85-100 degrees

Hello,

I got given this computer about a year ago. to my understanding its a decent set up, used to run all my games like the witcher 3 for example, on the highest graphics and had no problem. a couple months ago i took it apart to clean for the first time, keep in mind my knowledge with computers is pretty limited. once putting it all back together and turned it back on it could no longer run the games i had before. ive slowly been doing some research when i have the time and tried different things but i seem to be making no difference. ive tried multiple tools like speccy and i ended up learning the cpu was overheating majorly. i cant figure out which part might need to be replaced, whether it be my cooling system or not, i have no idea, at this point im genuinely at my witts end. i made sure when unplugging and taking out each part to mark what plugged in to where, so im pretty sure its not the wiring i ****ed up. all the fans are working still. i even tried replacing the thermal paste which i learnt how to do from a video, so that cant be it either. now, most times i turn on the computer it goes to a warning screen saying the cpu is overheating and the only thing i can do from there is enter BIOS settings or turn it off. from the BIOS settings i can still enter my computer but the cpu's heat slowly rises and doesnt stop rising till around 90 degrees even if im just using the internet or playing solitaire. someone please help