It happen that some thermal pads and thermal pastes after being under high temperatures, on cooling off harden like epoxy. And came off from board together with chips and anything else what is "glued" below them. There was photos with AMD video card broken in such way, can't find at the moment. OP, do not try to pull radiator and circuit board apart with force. You will damage your board beyond repair. What you must do - heat radiator with hot air to 60-70C and then very carefully wiggle it away. DO NOT TEAR IT WITH FORCE. If radiator won't budge, try to find what is holding it. Forgotten screw or something. Obviously use gloves to avoid burning your hands.
Also before radiator removal get replacement thermal pads. One who are between radiator and RAM chips. Most likely you will need them.