You might try copying your map data files (mapdata directories) to your external memory card, then format your internal card completely empty. Start Maps, and it should recreate all the directories etc. to the internal card. After that you can copy the mapdata directories back to internal card where they used to be. (Remember to write down the path before formatting...) Most likely you need to re-enter your license data/number.

If you encounter the problem on copying files... then the mapdata file(s ) may have corrupted and is the root cause for your Maps crashing.

WARNING: If you had voice guidance, the voice files will also be lost and you'd need to save them to external memory card as well. I do not use routing nor voice guidance, so I can not tell where they are stored.

I did encounter similar Maps problem couple weeks back when traveling in Scandinavia: Maps worked fine until once after closing the app and reopening, Scandinavian maps were not shown. Luckily I had backup copy of my mapdata and I just simply deleted my Scandinavian maps in Maps application. Then I copied the mapdata back from external memory card (using file manager), and I got my Scandinavian maps back... Don't know what caused this.