Ok, I have fixed it - I (backed up) and then manually cleared out the /cache directory except for the index.php.
After that - presto, site is back up and running.
What I suspect happened is that the cache or something went crazy and wrote out using all of our disk space...but leaving a corrupted file in the cache.
After reboot, it was trying to load up a half-written cache file and that is what just produced the blank screen instead of the msg boards.
Anyway, wanted to post the resolution in case any one else hits it.
