Is the heatsink and CPU now successfully detatched? If so, the gooey sticky thermal pad mess on both the heatsink and CPU need to be cleaned using q-tips (or in the UK- cotton buds) and special cleaning fluids like isopropyl alcohol or some type of safe Thermal Interface Material cleaner (such as the oil free Akasa TIM Clean). Lumps of thermal material should be removed using a sharp edged tool (with safety).
The thermal interface material is supposed to fill the valleys in both the microscopic surfaces of the heatsink and CPU die. It heats up and becomes gooey with heat and solidifies when cool.
The CPU should be cleaned each time you replace the heatsink (as i've learnt
). Remember to replace the thermal interface material when you do remove the heatsink- never add thermal paste / pads ontop of old thermal interface.