If you disassembled the laptop in the right order and then replaced only the thermal paste on the cooling assembly that makes contact with the processors die, then reassembling the laptop in reverse order of disassembling should've lead you to a working laptop.
Thermal tapes should not be replaced with thermal paste unless the gap between heatpipe assembly and the heatsource is very small which can be covered up with a thermal paste.
Make sure that all connectors are in the right order. Did you forget to reconnect the CMOS battery?
Also with the sticks of ream removed and the laptop disconnected from the wall as well as the battery removed, press and hold down the power button for 30 seconds. Replace the stick(s) of ram in the laptop and perhaps swap their places, then power using the wall adapter(not the battery) to see if powering the laptop works