I can add what the Digital Ocean support team told me. Namely, the server suffered from some type of "kernel panic" that is very rare. The Digital Ocean support team do not have access to our back-end user interface nor can they access our server otherwise.
They guided me, step by step, through several attempts to restore the server from this "kernel panic". We made eight different attempts which all failed and we don't know what caused this issue in the first place.
Luckily, we have server backups enabled and this was the last resort.