First of do not turn on your PC again in this state.
I would advise to disassemble, check the thermal pads again and redo GPU thermal paste. I am
guessing the GPU is not making enough contact with the heatsink, causing overheating and fans at 100% trying to compensate (or some horrible instant damage ESD event, but that is far fetched).
Maybe this is of some help, don't know if you already tried some of it:
1. thermal pads from the kit - are you sure these are in the right place? Did you remove the protective foil?
2. existing thermal pads - are these still where they should be?
3. heatsink - is the heatsink making proper contact with the GPU? And with every thermal pad? Are the four screws holding the thing to the PCB tightened enough? (don't overtighten either). Are there any obstructions preventing contact?
4. GPU - you would need to remove the heatsink and see how the paste has spread. Does it look like the paste has made proper contact with both GPU and heatsink? Clean it, re-apply paste (either the rice grain method or the full thin spread method should be fine) and re-assemble it (tighten the screws cross-wise and every screw a little bit at the time until they are all tightened enough - this is to get even pressure and spread of the thermal paste).
When in doubt contact tech support!