Maemo mapper typically looks for maps in a directory named Maps it will build this directory by default in the device memory where the Documents and Audio clips directories are found.

You may simply be out of device memory. In my experience if the Maps directory can not be found by M/Mapper or a file is corrupt, M/M will still open to a black screen and you could still access the menu.

I would check memory availability by using the control panel applet...

EDIT: BTW, it is easy to get to this device memory full state if you were downloading large amounts of maps and had not changed the storage path to a memory card.

If this is the case the simple fix would be to move this Maps directory from device memory to the external or internal storage cards using file manager, fire up M/M that may now open to a black screen (no maps), then change the file path by browsing to where you moved Maps using the Manage Repositories screen that I directed you to in my first post in this thread.