I joined the server in spawn and tried to teleport home. It said the territory had been unclaimed and I wasn't sure why. It's just frustrating to see my hard work over the past few days go to waste lol.
This sounds like your land was unclaimed because you did not have enough money in your faction. Make sure to deposit money in the faction, so when you are taxed, your land is not unclaimed.
Apart from what everyone else said, you may have been overclaimed / pillaged. This means that if you have less power than land, players can unclaim / overclaim your land, starting from the border of the claim.