Bug: all zwave devices fail or all Zigbee devices fail
Description of the Issue
After the V2 hub update on 17 November 2015, an entire set of network devices will become unavailable. In some cases, all of the correct commands are sent, but the devices appear not to respond. In other cases there are no command entries in the logs. Sometimes the devices can be manually toggled from the mobile app, sometimes not.
Note that there is a separate known issue with similar symptoms caused by having more than one Sonos device attached to SmartThings. 
Regions affected: US definitely, unsure about UK
Phone OS affected: all
Variable or Always Occurs: Variable, but for some users appears to get worse and worse.
Devices should respond to commands whether from routines, smart apps, or the mobile app.
SmartThings official response
Engineering is aware that there is a problem for some users and is looking into it.
There is a separate problem with the same symptoms caused by a manufacturing defect in the power cord. That one requires replacement of the hub and only affects a few people.
Some users report that if they either reset using the reset button on the hub, or power the hub down completely for 15 minutes (including Removing the batteries) that then they will get control again for a while, but it may be as short as 10 or 12 hours before the problem occurs.