cannot read block errors !

Hi All;

we're getting these errors on our system today:

Sep 9 00:53:45 ux-rqs04-atl2 ri_load[5846]: Caught Exception!
Shutting down: ERROR: could not read block 703769 of relation
37698/17627/38565: Input/output error CONTEXT: SQL statement "SELECT
1 FROM ONLY "our_schema"."big_table" x WHERE "id" OPERATOR(pg_catalog.=) $1
FOR SHARE OF x"

Sep 9 15:34:36 ux-rqs04-atl2 ri_load[23717]: Caught Exception! Shutting
down: PANIC: right sibling's left-link doesn't match: block 1377627
links to 3120580 instead of expected 1736900 in index "no_dups" CONTEXT: COPY
big_table, line 1563: "157945230 x/1739441607/false/p_1378267744=0"
\copy: unexpected response (7)

Anyone have any ideas what's gone wrong and how to fix it?

Thanks in advance