Thanks for all ideas, everybody!
@Punkrockaddict: could you make some pictures for me? That would help a lot, I think.
@sfbp: I didn't have the famous "unlock trick" as far as I know. The only thing I did was an update and at the same time I got a message for an updat for Java and I did this update at the same time. After a few minutes I read a fault message (I don't know which message) about not being able to complete the update. And immediately There was the message on my MZ-NH10 "No disc".
I tried more than 10 discs. No result.
Should I trie to use a screwdriver to open the unit?