I've 6934 customers, and very often when I try to send indivudual email to specify customer, I've this error:

1317 Query execution was interrupted
in:
[SELECT DISTINCT c.customers_email_address as customers_email_address, c.customers_lastname as customers_lastname, c.customers_firstname as customers_firstname FROM customers c LEFT JOIN orders o ON c.customers_id=o.customers_id WHERE o.date_purchased IS NULL ]
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2006 MySQL server has gone away
in:
[select count(*) as total from sessions where sesskey = 'e70b08bdd55fed393b593d4908bb3206']
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What happens?