The server ran out of disk space.
I've asked the host to upgrade the VPS ... but that won't be instant.
(It will also cost more) which will give us more space, and RAM (so should help performance too)
I'm also going to look at alternative options for user files, as they are all just on the server right now.
Apologies for the outage ... but it might happen again until I get us more space sorted.
Looks like the cheapest viable solution to more storage ... but we're far from having 1TB (which is the minimum) of stuff to store, or $60 a year to throw at it (personally)