[EndlessDissent]

If you're using MIUI, 3rd party apps are known to break Wifi tethering for a lot of people. Use the native feature. Settings -> System -> Tethering & Portable Hotspot. Just turn it on and configure it.

 

[EndlessDissent]

Have you uninstalled the app? If not, do that, and try again. I'm not sure why it would lock up your entire phone.

 

[EndlessDissent]

You're not supposed to reinstall the app. You're supposed to uninstall it completely and try the built-in feature. If the built-in feature still doesn't work, I'd try a completely fresh install of MIUI without installing any tethering apps to see if it will work at all. I've never personally had a problem with the native MIUI tethering feature, so I'm willing to bet that having the app installed is causing your problems.

 

[EndlessDissent]

Why would you lose your phone? If you're worried about your data, back up everything with the MIUI Backup app, or if you're planning on trying other ROMs, back up your apps with Titanium Backup. Then you can do complete data wipes and then restore everything when needed.

Nothing I told you to do will hurt your phone or data if you use a backup app. It's completely normal troubleshooting to see at which point the problem occurs. If you do a compete wipe (data, cache, Dalvik cache, and if you're using Amon RA, it's a good idea to wipe boot and system also), you should be starting from a completely working state. That way, if an app is causing the problem, you'll know because tethering will work until the app is installed.

Before you go through all that trouble, though, check to see if your tethering works while the app is uninstalled. Then, you won't have to worry about your data.