If the query is a single-table query with a WHERE clause, the problem can be an overly restrictive WHERE clause that eliminates the whole table.
If the query is based on updating via a JOIN, it is possible that nothing matches the OUTER JOIN requirements.
If you have warnings disabled, it could be a syntax error in the query itself.
Without knowing more about the query and the surroundings, it will be EXTREMELY hard to diagnose this problem.