It sounds like you accidentally removed the contents in the databases. No worries though just run the following commands in PHPMyAdmin inside the tables.
INSERT INTO `cron_task_runs` (`id`, `task_id`, `company_id`, `time`, `interval`, `enabled`, `date_enabled`) VALUES
(14, 14, 0, NULL, 1440, 1, '2012-01-01 00:00:00'),
(9, 9, 0, NULL, 720, 1, '2012-01-01 00:00:00'),
(17, 17, 0, NULL, 1440, 1, '2012-01-01 00:00:00'),
(13, 13, 1, '00:00:00', NULL, 1, '2012-01-01 00:00:00'),
(1, 1, 1, '23:00:00', NULL, 1, '2013-08-11 10:15:52'),
(15, 15, 1, NULL, 5, 1, '2013-08-11 10:15:52'),
(16, 16, 1, NULL, 5, 1, '2013-08-11 10:15:52'),
(18, 18, 1, NULL, 5, 1, '2013-08-11 10:15:52'),
(19, 19, 1, NULL, 5, 1, '2013-08-11 10:15:52'),
(12, 12, 1, '20:00:00', NULL, 1, '2013-08-11 10:15:52'),
(11, 11, 1, NULL, 1380, 1, '2013-08-11 10:15:52'),
(10, 10, 1, '12:00:00', NULL, 1, '2013-08-11 10:15:52'),
(2, 2, 1, '23:15:00', NULL, 1, '2013-08-11 10:15:52'),
(3, 3, 1, '11:15:00', NULL, 1, '2013-08-11 10:15:52'),
(4, 4, 1, NULL, 5, 1, '2013-08-11 10:15:52'),
(5, 5, 1, NULL, 5, 1, '2013-08-11 10:15:52'),
(6, 6, 1, '16:00:00', NULL, 1, '2013-08-11 10:15:52'),
(7, 7, 1, '19:00:00', NULL, 1, '2013-08-11 10:15:52'),
(8, 8, 1, NULL, 5, 1, '2013-08-11 10:15:52');INSERT INTO `cron_tasks` (`id`, `key`, `plugin_dir`, `name`, `description`, `is_lang`, `type`) VALUES
(1, 'create_invoice', NULL, 'CronTasks.crontask.name.create_invoice', 'CronTasks.crontask.description.create_invoice', 1, 'time'),
(2, 'autodebit', NULL, 'CronTasks.crontask.name.autodebit', 'CronTasks.crontask.description.autodebit', 1, 'time'),
(3, 'payment_reminders', NULL, 'CronTasks.crontask.name.payment_reminders', 'CronTasks.crontask.description.payment_reminders', 1, 'time'),
(4, 'apply_payments', NULL, 'CronTasks.crontask.name.apply_payments', 'CronTasks.crontask.description.apply_payments', 1, 'interval'),
(5, 'provision_pending_services', NULL, 'CronTasks.crontask.name.provision_pending_services', 'CronTasks.crontask.description.provision_pending_services', 1, 'interval'),
(6, 'cancel_scheduled_services', NULL, 'CronTasks.crontask.name.cancel_scheduled_services', 'CronTasks.crontask.description.cancel_scheduled_services', 1, 'time'),
(7, 'card_expiration_reminders', NULL, 'CronTasks.crontask.name.card_expiration_reminders', 'CronTasks.crontask.description.card_expiration_reminders', 1, 'time'),
(8, 'deliver_invoices', NULL, 'CronTasks.crontask.name.deliver_invoices', 'CronTasks.crontask.description.deliver_invoices', 1, 'interval'),
(9, 'backups_sftp', NULL, 'CronTasks.crontask.name.backups_sftp', 'CronTasks.crontask.description.backups_sftp', 1, 'interval'),
(10, 'suspend_services', NULL, 'CronTasks.crontask.name.suspend_services', 'CronTasks.crontask.description.suspend_services', 1, 'time'),
(11, 'exchange_rates', NULL, 'CronTasks.crontask.name.exchange_rates', 'CronTasks.crontask.description.exchange_rates', 1, 'interval'),
(12, 'deliver_reports', NULL, 'CronTasks.crontask.name.deliver_reports', 'CronTasks.crontask.description.deliver_reports', 1, 'time'),
(13, 'cleanup_logs', NULL, 'CronTasks.crontask.name.cleanup_logs', 'CronTasks.crontask.description.cleanup_logs', 1, 'time'),
(14, 'backups_amazons3', NULL, 'CronTasks.crontask.name.backups_amazons3', 'CronTasks.crontask.description.backups_amazons3', 1, 'interval'),
(15, 'process_renewing_services', NULL, 'CronTasks.crontask.name.process_renewing_services', 'CronTasks.crontask.description.process_renewing_services', 1, 'interval'),
(16, 'unsuspend_services', NULL, 'CronTasks.crontask.name.unsuspend_services', 'CronTasks.crontask.description.unsuspend_services', 1, 'interval'),
(17, 'license_validation', NULL, 'CronTasks.crontask.name.license_validation', NULL, 1, 'interval'),
(18, 'accept_paid_orders', 'order', 'Accept Paid Pending Orders', 'Automatically accepts paid pending orders if the order form allows it.', 0, 'interval'),
(19, 'poll_tickets', 'support_manager', 'Download Tickets', 'Connects to the POP3/IMAP server to download emails and convert them into tickets.', 0, 'interval'),
(20, 'close_tickets', 'support_manager', 'Close Tickets', 'Automatically closes open tickets as configured for departments in the Support Manager.', 0, 'interval');