The party suddenly dies without reason

This is part of the Technical Documentation - Copy Protection in Dungeon Master and Chaos Strikes Back for Atari ST as well as in Dungeon Master for Amiga version 2.x.
If the game does not detect an original floppy disk then one of the possible consequences is to instantly kill the party and then hide the 'Restart' button. When you die normally, the 'Restart' button is always shown.