both recursive-only BIND9 went deaf until rebooted

[lconrad at go2france.com]

fbsd 8.2 VM with BIND 9.9.5

fbsd 10.0-RELEASE VM with BIND 9.10.0-P2

the older machine had uptime of 400+ days, the new machine only a 
couple weeks

24 hour query logging shows several million queries/day

At about the same time last night, both stopped answering queries 
until rebooted.

before reboot,

load of about 1 (we see elevated load alerts with ssh brute force 
attacks)

memory not swapping, plenty of free MBs.

nothing in syslog,

no sign of ssh brute force, ssh worked

rndc status showed ok

sockstat -4 showed  bind listening on :53

all DNS queries from outside the machines timed out

ssh shell command:

"dig @127.0.0.1 domain.tld any"  answered normally

What other forensics could have been checked?

thanks
Len