Invalid page header in block 211 of relation

Hi List,

I'm using postgresql 7.4 on FC4 linux.
i have one table named "purchase". it is working fine since 10 months but
suddenly from 3-4 days it creates problem.
when i'm fire a simple "select * from purchase" query on psql prompt,it
gives following error:

Invalid page header in block 211 of relation "purchase".

i cannot understand what is the problem, why it is giving this error.
plz. help me.

Thanks
Ashok

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