Ok, I got it back. The nginx service refused to start. Thanks to your help, I used omv-firstaid to reset the WebGUI http port to 82 and restarted nginx and this time it worked. I was able to log into the WebGUI and undo the website stuff. Everything is back to normal.
The thing that threw me off was that I had set the WebGUI to SSL-only, so I didn't think that using Port 80 for something else would cause any problems, but it seems that it screws up nginx's config to have two websites on the same port, even if one of them is disabled.
Anyway, maybe there could be a small warning text in the openmediavault-websites plugin to warn users not to use a port number that is already used by the WebGUI. I guess I learned my lesson
Just a note to say that the support from this forum is what makes OMV the best NAS distro out there. Thanks guys!