The board in the Amazon link looks a little different physically, but going by that picture, the retention bar holds the heatsink in place. If you look diagonally up and to the left of the bared chip in your last picture, you can see an inverted U shaped metal clip set in black plastic. To the lower right of the chip, there's a bare space on the board with 2 small holes. I'm guessing it used to have a matching U clip that popped out. The retention bar hooks to both of them, probably with some bend to apply pressure.
Thanks for the response I will look for the other hook that came off, should I put new thermal paste on the heatsink?