Just found this thread and it does seem that the 8.1 update that I got for my 8x on Tmo has broken this. Was thinking of doing a device reset but it looks like that won't help.
If you're resetting and still in 8.1, you may have the same issue that I had with my Lumia 925--basically, internet sharing only works in 8.0, and no versions of 8.1.
But did you actually have a T-mobile phone? If T-Mobile rolled out an update that broke internet sharing, then they'd really screwed up big-time, but I hadn't heard that 8.1 had gone out on the 8X. If you've got an unlocked phone, like me, you may be out of luck (since I don't know if HTC has a tool that can downgrade from 8.1 to 8.0). Nokia's tool only works for 8.1, not 8.1.1 (as it's called).
If it
is a T-Mobile phone, and not unlocked, you have a case for a replacement, at least.
Reading through this thread, it really doesn't sound like any of these solutions apply to T-Mobile (just AT&T, and that's if you're lucky, and they may be temporary). As they seem to involve a phone wipe anyway, I'm very reluctant to try them myself.