1. First check if you are on the right server. We often have two clusters running, the Valhalla Cluster & the Season Valhalla Cluster.
Press the "ESC" key ingame and read the server name you are currently on.

2. If you are on the right server open your tribe log with "L" and read what it says. If you were offline too long, your base will disintegrate.
In the tribelog it says "Decay". If an admin shot down your base, you will see the name of the admin and what he destroyed.
If these answers don't help you, please open a ticket in the Discord.

You can find the link to the Discord ingame under F1.