The problem was a typo that had cropped up in /cvs/cds/caltech/chans/daq/C2ATF.ini when I was editing this file. I had to fix that file and reboot oms to get the frame builder working again. Now it appears to be fine - I can telnet into fb0 again.
I tried to reboot the frame builder last night, as below, and now it won't restart and I can't log into it. If I can find the box then I can try to physically reset it.
[controls@ws1 ~]$ telnet fb0 8087