Sounds like it is all you, which is a bit pointless, as does the fact that you are hardly playing, which could be another factor, if it isn't all you.

 If the chief isn't playing, there is not much reason to stay in  such a clan as it's all work and little to show for it.

If it is all you, just  try logging in to your ''clan'', other yous, you may find they are still there, and you can reassemble ''the clan'' .

Sounds like you accidentally clicked on "Disband the Clan".

I am not sure what you mean by gradually building up the clan - perhaps just starting a new clan will get you back to where you were.  Otherwise try a message to support, they might restore the clan for you.

https://vikingsweb-support.plarium.com/web/en/tickets?game=vikings

OK, I will add this which directly answers what probably happened.

Plarium recently  bumped all non active players from all clans.

Ah, thank you. That must be what happened since I'm just now signing in months later.