Filter Stuck on Lens

[Elmo]

Try using a rubber band so you can grip without squeezing the CPL.

[Raoul Isidro]

The danger of trying to pry open the stuck filter is breaking the proboscis part of the lens from the main barrel. This is the part that moves freely when focusing or zooming. Never grab the lens by the main barrel when forcing the filter, as you are actually stressing the lens components! Zoom or focus the lens so that the proboscis is out of the barrel and hold on to this part and not the lens barrel.
A few drops of warm water around the threads would loosen any salt that has formed. These crystals are the ones that make these filters stuck. (did you go to the beach lately?) Then gently, gently, twist free the filter. Finally, exhale with a sigh of triumph...
Cheers!