After updating civi 4.6.15 to 4.6.16 no access to contacts
I looks like CRM-18567 and CRM-18504

Have a look at Chris Burgess comment, there are couple of commits that seem to fix the issue.

https://github.com/fuzionnz/civicrm-core/commit/a0bcc16171d2f30c4416aeec079bab56c313390a
https://github.com/fuzionnz/civicrm-core/commit/d3e9ab21a0f2256197125854fe6312c3fa0c8c8a

I would recommend to create a backup or replicate your installation in a development environment, apply the commits mentioned and check if it solves the issue.