If the cover hasn't come off before, it's probably just sticking. There should be a couple of tabs at the bottom of the cover that hold it in place. The whole thing comes off but kinda rotates at the bottom. You should be ok to put a bit of force pulling it off, just be sure not to snap off the tabs at the bottom. It should come off like a dishwasher door. Once it starts moving, you can pull it upwards to get the tabs out of the holes. I hope this helps. If you really have trouble, I'll send some pics.
When you finally get it off, post some pics of the old filter. I'm a bit curious to see how gunky it is.