I had the same problem, my Personal Hotspot tab on my wifi+cellular iPad Air just disappeared after upgrading to iOS 8. I tried everything from turning my data on and off, LTE on and off, roaming on and off, reset APN settings, reset all network settings, turned the iPad off and back on, removed the SIM card and reinserted it...nothing worked until I went into Settings > Mobile Data > APN Settings and set my carrier's APN setting the same under the Personal Hotspot heading as it was under the Mobile Data heading.
Immediately after this my Personal Hotspot tab reappeared beneath Mobile Data and I was able to use my iPad air as a hotspot again. Hope this helps. Just for the record I use Vodafone in India as my service provider and the correct APN setting under both Mobile Data and Personal Hotspot is "iphone"
Edit: Try entering anything under your "Personal Hotspot" APN Settings and it should give you back your Personal Hotspot tab. Then all you need to do is turn the Personal Hotspot back on again!