The database server cannot read the disk page that contains a row of a table. Check the accompanying ISAM error code for more information. A hardware problem might exist, or the table or index might have been corrupted. If the query was using the dirty read isolation level, this error code may be normal behavior caused by reading data that was in a temporarily inconsistent state from a concurrent update on the same data.
Unless the ISAM error code or an operating-system message points to another cause, run the oncheck utility (secheck with IBM Informix SE or tbcheck with IBM Informix OnLine) to check and repair table and index.