You could check to see if your neighbor had a
survey done before building the garage. That survey
could have been in error, or perhaps the markers were moved.
In either case it's reasonable to contact the surveyor.
If no survey was done, and if you still think
it’s over the line, you will probably need a boundary survey
to determine where your property lines fall on the ground.
correct based on what your surveyor determines, or even if the building
or fence is merely closer to your boundary than is legally permissable,
it's likely you'll want
to contact an attorney to discuss the options and ramifications.