First, you must be absolutely certain of the property boundary lines. Review your legal description of the property in your deed. If the boundary issue is clear from the description, you will be able to settle the issue quickly.
Also, you can file a claim to quiet title, which will have a court resolve who owns the disputed land. Before you file, talk to a real estate attorney to make sure you have exhausted options to negotiate a settlement, and to confirm that you have evidence to back up your claim.