Failure to boot after re-applying thermal paste

Try resetting NVRAM/PRAM. First, turn off your Mac then follow these steps:

1. Press the power button.
2. Immediately press the Command+Option+P+R keys.
3. Hold the keys until your Mac restarts and you hear the startup sound, keep holding till you hear the startup sound a second time.
4. Release the keys.

You could also try an SMC reset:

If the battery is removable:

1. Turn off your Mac.
2. Remove the battery.
3. Press the power button and hold it for a few seconds.
4. Reinstall the battery.
5. Press the power button to turn on your Mac.

If the battery is non-removable:

1. Shut down your Mac.
2. Press Shift+Control+Option on your keyboard and press the power button at the same time.
3. Hold these keys for 10 seconds.
4. Release the keys.
5. Press the power button again to turn on your Mac.

Other than the above, make sure all your cables are connected and fully seated, make sure all the screws are in their correct holes, so you do not cause damage by putting a long screw in a small hole :(

Hope this helps :)

@nobert - Yep, you over did the paste! You'll need to clean it all off. sadly its going to take a bit more than wiping off as you’ve got it over the small SMD devices that are also on the green chip carrier. Using a soft tooth brush and 90% isopropyl alcohol carefully clean off the caught paste around the SMD components. Use extreme care as you don’t want to lift any of them off which is quite easy!

Clean the heat sink as well asd then use one quarter of what you used! It’s not the quantity it’s the placement and not over doing it thats important only getting it on the Black silicon chip.