While I understand this is frustrating, an un-managed server is not something you can “set and forget”. While I don’t know the specifics here it sounds like your droplet became infected or in some other way began generating abuse complaints against your account.

Failure to respond or take action on abuse complaints leaves our team with little choice but to terminate an account. For some types of abuse, if our team fails to act we could become legally liable for the actions of your server (I’m not a lawyer and this is not legal advice, consult a lawyer for specific information on how the laws apply).

As I said, I don’t know the specifics of your particular account or situation nor am I accusing your of any negative intent or bad actions. Based on your description, this is the most likely and common scenario.