Does the nav device have any software loaded?
If no OS is loaded it won't do anything.

What software is loaded?
Tell us the version - top right hand of the screen when in 'Set' (or 'Settings', depending on what language you have loaded)

Is the map disc the right version for the Mk3?
The Mk3 cannot read discs made for newer cars like the E90 with the MOST system

Is the map disc an original disc?
When the laser gets old/dirty, the Mk3 can struggle to read a copied disc, and it will eject it

Is the Mk3 actually spinning the disk?
Have a listen, you will hear the spindle motor turning when the disc is inserted if all is well.