I've never had that heart-stopping moment when a filter fuses itself to a lens, but this tends to happen to people who are currently shooting a lot more than I am. Whether due to heat, impacts, or cross-threading, things like this will inevitably happen when a delicately-attached accessory with fine tolerances decides to become a permanent resident on your glass. What does Canon suggest? Well, according to Craig Pulsifer on his blog, they say: get out your hacksaw and ballpein hammer.
Copyright Craig Pulsifer
If you find your lens filter has become stuck, and you have the stomach to take such blunt instruments of destruction to it, first use the hacksaw to cut down to the outer rim of the filter to the glass surface, being careful not to cut into the lens. Canon wasn't specific on this, but I'd personally try to get a hacksaw with a fine-tooth blade for more precision. After that, tap the filter glass with the ballpein hammer in progressively stronger increments until the glass breaks, at which point you'll need a pair of pliers to remove the bigger bits of glass and an Air Blaster of some sort for the tiny particles. Finally, use the pliers to bend the filter's thread frame towards the center of the lens, which takes pressure off of the lens threads.
Sound a little scary? Perhaps you'd like to try our idea, which is a bit more precise. Coffee and cake are optional but recommended. Of utmost importance: all lensmakers warn against using any "assisted torque", like a jar or bottle opener, as doing so could stress and permanently ruin the threads on the lens.