|Did you have the power cord unplugged when you were messing around?|
The heatsink MUST be installed properly or the CPU could possibly overheat & fry. Make sure it's not backwards:
Also, you need to use the proper amount of thermal paste between the CPU & heatsink. Just a tiny dab spread thinly & evenly on the CPU core is all it takes. Using no paste, too much paste, or re-using old paste *may* cause the system not to boot.