My MGC does that if I don't crank it about once a month when it is off the road and I have to do what Doug said to get it free again. I expect this proclivity varies with the amount of ferrous metal used in the friction surfaces of various makes of disc.
BTW, I would NOT advocate hitting the centre with anything as proposed above - you will force the disc to flex in a way it isn't intended to flex and create a possible failure situation if you ever wanted to use it again.
If you don't intend to use it, get a wood chisel and hammer and go art it around the edges (bevel upward and angle it away from the flywheel face). If you do want to reuse it, give it a spray with a solvent that may help when it seeps in between the disc and flywheel (obviously you can't use anything nonvolatile that will stay in the friction material), and start in gently prying with that chisel or a very thin shim, all around the edge. You may or may not be successful. Best way was to have done it when the engine was running, but we don't always find out about it then.