Well, this is only a guess, but I'd say the forum core directory has become corrupted or subject to malicious attack? There is also a possibility that a phpbb forum upgrade attempt has failed, or something else trivial ??

We can only guess until the admin actually confirms what's happened, but I always had my suspicions that the forum at one point was hosted on a local machine in someones house other than a dedicated server. This might explain how it was terrible slow on occasions. I do know there is Cloudflare somewhere in the chain.