Shut down the computer and then power it back up after waiting 10 seconds. Disconnect all peripherals except those needed for the test. If the problem is still there, try booting into the Safe Mode using your normal account. Isolating an issue by using another user account
The intent is to see if it is specific to one account or a system wide problem. If Back-to-My Mac is selected in System Preferences, the Guest account will not work. Try setting up another admin user account to see if the same problem continues. You have no backups while you were running 10.11.5? If you revert to Yosemite and then install El Capitan, you will get 10.11.6.
