iPhone Personal Hotspot via USB stopped working after OS X 10.10.4 update
The reason for USB tethering not working is in 99% cases bad iPhone driver - yes, even on OS X. It may sound weird, but going to http://www.apple.com/itunes/download/ and re-installing iTunes from the latest iTunes installer will probably solve the issue for you. Sometimes the driver can get corrupted during a system update etc. Give it a shot and report back :)