Blaupunkt DVD-MER display says no disc but ther is a disc in the unit

This problem means that the system doesn't recognize the presence of the disk.this could be because the the optical lens is dusty or worn out.you can manually clean the lens by opening the top cover then use a cotton buds moistened with an alcohol (rubbing alcohol) and gently wipe it on the lens.
The second that causes same kind of problem is a defective spindle motor.This motor is the one holding and spinning the disk.replacing this motor could solve the problem .If you are handy with electronics you can replace it by youreself or you can ask the help of your of an professional...