Edit: I'm pretty sure the 2 above solutions will NOT work since this is a stand-alone device.
I had the exact same problem, came here through Google and thought I'd type a reply with my solution.
I burned a UDF DVD, inserted it into my Philips 3450H, and it kept trying to load it, and load it, and load it. Switching back between TV tuner and DVD mode, making way too much noise, and no matter what I did it would NOT eject the disc. (And I agree that the OPen/Close button SHOULD override everything).
I unplugged (!!) the DVD/HD recorder, unscrewed the lid (you need those star shaped screwdriver bits for this).
Inside you will find a black "DVDROM" device. the part closest to the front of the player had kind of a 'lid' with a white circle inside.
Unscrew the small metal strip/plate that holds the lid in place (4 screws), uncsrew the black 'lid' part, and you will see your disk.
Remove DVD disk and throw it out. Put everything back together, and in my case the thing is working again.
disclaimer - use above at your own risk