it happens.... depends on temps, paste, and age... but there is always some degree of "stickiness" involved and the amount of force involved will vary (ask Lord, he removed his current Winchester 3500 with cooler and had the "straighten bent pins" fun!)
My suggestion would be to proceed as follows...
Wiggle the heatsink in all possible directions WITHOUT pulling - IE a little diagonaly twisting shimmy within the confines of the socket set. There really should not be enough scope for movement to risk any pin damage, but there MAY be enough to loosen the bond.
After you have done that and assuming it has not come free readily in the process the next thing you are forced to do is simply pull!
When you do this try to pull directly upwards and to support the board so you are purely pulling the cooler from the socket (not flexing the mobo, which is bad news).
Trust me eventually your zif socket will give up the struggle and release the pins - so long as you pull straight and do NOT let the thing go flying across the room all pins SHOULD remain intact.
So now you have a heatsink with your CPU stuck to the bottom, right? Here you can get a little more creative and hopefully have something sharp and tough to prize into the gap (I found a steak knife worked for me, but you can improvise)... if you have trouble you can try applying some isopropyl alcohol to the joint to loosen things to work it in. then twist and PLICK the paste will give it's grip up and if you are unlucky the CPU flies across the room to give you a thrill of the worrying kind, or hopefully you got the pressure and direction right and it just lifts off.
watchwords are do not panic
do not be afraid
prepare and consider each step
What you up to anyway - CPU upgrade? Just being nosey...
