|As kuwese said, a CPU is a chip that fits in a socket on the motherboard. What you did was open the computer case.|
"put everything back together"
Does that mean you actually removed parts for cleaning? if so, what did you remove? Did you unplug the power cord 1st? Did you unclip the heatsink/fan from the CPU? If you did, & didn't apply fresh thermal compound, that is most likely the problem. However, if you used a vacuum rather than compressed air, there may have been a static discharge from the hose that damaged something.