It sounds like someone over tightened it and/or used a gasket that was worn out. I did that once on mine, fortunately the lid had instructions printed on top on how to open it. Get a piece of wood (such as a 2x4) and put one on either side of the "handle" on top of the lid. Use the piece of wood to turn the lid. Next time you replace it, don't tighten it as much. Your lid might not be designed the same as mine in order to accommodate the 2x4.

Other things that should help (based on this thread: http://www.poolforum.com/pf2/showthr...sket-lid-stuck ) would be to try and loosen it slightly while the pump is on, open the pressure relief on your sand filter, or pour some hot water on it. Although hot water tends to make things expand so I wonder how well this would work

It might be time to replace the gasket there as well. When that gasket and the filter gasket get worn the lids get hard to remove (especially if you lube 'em up after they are worn out). I learned this the hard way.