|Hmm....what kind of CPU do yo have?|
When you applied the thermal paste, was the past white or silver? Silver is usually conductive, and can short out a processor if cross contact occurs. White is usually ceramic and is less conductive. It's possible your CPU could have fried.
It's best to have a spare to troubleshoot.
Did you apply the paste similar to a grain of rice, and spread evenly across the processor?
Ensure all cards are seated correctly, all cables plugged in correctly and all fans are spinning correctly.