Icinga, probably the best known Nagios fork, and it's database connector ido2db sometimes don't come up correctly, especially after a "sudden death" like a from a power outage or an unsafe virtual server stop.
These steps should alway bring them up again:
/etc/init.d/icinga stop /etc/init.d/ido2db stop rm -fv /var/lib/icinga/ido.sock /var/lib/icinga/idomod.tmp /etc/init.d/ido2db start sleep 5 /etc/init.d/icinga start