If you removed the heatsink to renew the paste then you will have removed the CPU die as I did. There is only one right way of refitting the CPU as it has notches that match others in the socket, so they must match
But you also have to use the pins on the clamp that sits on the other side of the board to help locate the heatsink. Then once located it must be held in position until you can get two screws in to keep it there - not done tightly - just enough to prevent anything moving.
This was my problem. The heatsink moved slightly while I was trying to get the first two screws in. But I carried on not realising that the die had also moved.
Once I found out the Mac would not turn on, I dismantled it again to find I had slightly bent the CPU die.
Fortunately, an identical replacement with the correct socket code ultimately fixed it and the Mac now works faultlessly.