The old 400k drives are notorious for having the grease gum up the eject mechanism. Usually if/when this happens, you'll find that applying constant pressure with the paper clip will result in the eject mechanism slowly releasing the disk.

Whether the disk can be removed this way or not, the drives are not particularly difficult to remove from the computer. If it is gummed up with grease, get the disk out, take the drive outside and liberally spray the eject mechanism with contact cleaner until you've washed the grease out. The drive should then be "all good". It's probably not a bad idea to use some light grease on the various sliders and things, although in the day, the recommended fix was to leave the drive "un-greased".