select failure

Ideas where I should look for the answer to this problem & how I should fix
it?

observer=# select host, current_route, count(current_route)
observer-# from messages_200205
observer-# group by host, current_route;
FATAL 2: open of /var/lib/postgres/data/pg_clog/0534 failed: No such file
or directory
server closed the connection unexpectedly
This probably means the server terminated abnormally
before or while processing the request.
The connection to the server was lost. Attempting reset: NOTICE: Message
from PostgreSQL backend:
The Postmaster has informed me that some other backend
died abnormally and possibly corrupted shared memory.
I have rolled back the current transaction and am
going to terminate your database system connection and exit.
Please reconnect to the database system and repeat your query.
Failed.

--
Ian Cass