Physics is your friend (don’t break the plate unless you have to). While the suggestions to warm the pot make sense, practically speaking, you will likely burn the bottom of the empty pot if it has no liquid in it. I think that using leverage (i.e. the idea to stick a knife in between) is much more likely to work. Once you get it turned a bit, you can pull it out. Alternatively, you can put some water on top of the plate (± soap around the edges for lubrication) and place the whole pot in the freezer. After the liquid has frozen (to the plate as well), THEN run hot/boiling water over the bottom of the pan, flip upside down and bang out the plate.
The frozen water will expand the pot a little at the site of the stuck plate (frozen water expands). Then, when you run hot water over the pot, it will expand further, the water will melt a little, the soap will lubricate the edges and there will be just enough clearance for you to bang out the plate.
Of course, if all else fails, and the plate is plastic, and you can’t find a hammer, drill a couple of holes in the plate and pull it out using a wire hanger.