Stuck at boot screen after adding multi-user account
This procedure will reset your phone and hopefully solve your issue.
The sudden reboot is most likely the custom ROM's fault. Flash stable versions of ROMs to avoid crashes like this one.

Try this,


Unplug your phone from the power source and let the battery drain.
Now plug it in but don't switch on the device.
Follow the instructions here to enter Recovery mode on your Nexus 6.
From the stock or a custom recovery, do a full wipe (factory reset &amp; cache).
(Optional) Restore from Nandroid flash.
Do a normal boot.