Your map discs may be newer than what the nav can support.
Ensure the nav is at software level 3/1-63 - check the SW version shown in the top right hand corner of the screen in "Settings". Report what version you have.

If you have anything older than 3/1-63, then first upgrade the software to the latest (and last) version. Then try reading the maps again. Some old software versions don't work with the current discs.

If the nav unit still cannot read the map disc, ensure the map disc is OK by trying in another vehicle. Or try another known, good map disc.

If the map disc is OK but the nav unit still cannot read it, then it may be worth cleaning the laser. This is best done through dismantling and using a cotton bud with isopropyl alcohol direct on the laser lens, and being very gentle.

If it still doesn't work, the laser/nav unit is probably faulty.