If we have not been served with any papers
Can the people we are in the land contract, come in and change the locks?

Usually not BUT what does the land contract say about it?

What State?

Is it commercial or residential property

Where is it at, and what are the terms of the contract if you are in default. If you are in default, why,

It is a residential property.
It is in dowagiac Michigan.
We paid our mortgage every month (21)mos.
The taxes were not paid on time. And they took it upon themselves
To pay them. They told us we had to move.
But never sent us any papers, with a date or a chance to resolve
The problem. They are claiming we abandoned the house but we are there
Every day.

You need to see the contract in order to know if they are or are not within their rights.

Anything else is just a guess.

What it appears, you by contract where to pay the taxes on the property, When you did not pay the taxes, ( and I am guessing at what the contract says) but if you did not pay the taxes you defaulted in the contract. ( if that is part of the contract)

If this is like all of my contracts then if they declare you in default many times this changes you to a renter, so they can ask you to leave but will have to service you written notice, and can file for eviction in court