Unable to upgrade to 4.7.6
Ran into something similar. There are some database table alteration between 4.7.5 and 4.7.6 (specifically relating to foreign keys). These edited CiviMail tables. If you have used CiviMail extensively, these tables could be quite large. The ALTER TABLE commands can take a very long time causing time outs. 

The quickest option is to backup these tables, trucate them, run the CiviCRM 4.7.5 to 4.7.6 upgrade comands, and, finally, reimport the data.

Affected tables:


civicrm_mailing_event_queue
civicrm_mailing_recipients