Rookie mistake, I was transferring backup files to the machine when I filled it to capacity by mistake.
The device should have been fully up to date as of about 2 weeks ago, it is mostly used as a file server and is part of a Windoze 2012 R2 domain. The box still responds to a ping, but not the web interface. I'll be able to sit at the server tomorrow, I haven't done anything else to it today.
Maybe somebody has run into this before, any basic suggestions to try?