Personal Hotspot in Yosemite only shows up after unlocking iPhone
That doesn't match my experiences using iOS 8.0.2 and OS X 10.10 as I can initiate a request to start the hotspot while the phone is in my briefcase, locked, with the LCD screen off to save power.

Now, those requests often fail to connect properly many, many times - in fact, the success rate is far lower than the failure rate for me so far, but it has worked flawlessly from time to time.

I find that when a request to build up the tethering fails, I have the best success by getting the iOS device, unlocking it, navigating to the personal hotspot and toggle it off, wait at least 5 seconds, toggle it on and then cycle the WiFi on the Mac.

That almost always works to enable tethering for me on the current software versions. I suspect the reliability of this will get better with iOS 8.1 and future updates to OS X to have the requests work more often, but the design is clearly to allow requests at any time the devices are in range. If you can't even request the hotspot, perhaps you have a large amount of interference or the hardware of one device is not working properly.