WMZoo...
Try this...
1) Click anywhere on your desktop to get to what's called the Finder.
2) Click on the word "Go" at the top while holding down your "Option" key. This will show you the Library folder, which is usually hidden because it just has system stuff inside.
4) Select that entry, "Library," in the drop down menu under "Go"
5) You're now in the Library folder. About six folders down, you'll find "Application Support." Double click on that.
6) When the Application Support folder opens, now look for the folder called "Address Book." Double click on that.
7) When the Address Book folder opens, you'll see lots of files with system-like names. Search for "AddressBook-v22.abcddb"
8) Drag that file to the trash bin in your lower right corner. Empty the trash by selecting that option under "Finder" in the top menu. And reboot.
If it went right, your Messages app should now be working.
Good luck!