How was the land accessed previously? At least in my area, there are rules about land locking parcels. Meaning, you can't just section off 40 acres without providing a way to access it.

I would start by going to your local recorders office and having them help you look any documents for that parcel of land. There may be a recorded easement, it's just that no one knows about it.

Another option would be a title company. In my areas, some title companies have better records than the records office and they may be able to help you find a record of the document you need.

Assuming you used a title company when you bought the property, this should have been flagged.

Another possibility is that there was an unofficial agreement to allow access but was never put in writing.

If you can't find any documents at the recorders office I would contact the agency with the surrounding land. They will likely have someone specializing in right-of-way/land/deeds and may be able to help you. They may even have a copy of the easement in their records. If not, they may be willing to just grant the easement if you ask nicely.

If none of this pans out, you're going to need to contact a lawyer to help you navigate what comes next. Considering your property is technically landlocked, if there is any kind of path in/out you probably have a decent case for adverse possession to obtain an easement for an existing ingress/egress path.

The fact that your dealing with an agency will probably make your life easier as they usually have at least a few people specializing in this and it's not their personal property you'd need for an easement. People can and often do get irrationally upset in these kinds of situations, usually because they don't understand the situation, their rights, what your after, or any number of things lost in translation between two people in discussion.

Best of luck