Try restarting (turning off and back on) the phone and see if this resolves it. Sometimes simple works. If not, did you get a recent update? That could have removed the option. Also, it is possible that your carrier/service provider removed the option. Many are doing that. Others are limiting the amount of data, so that the option will turn off on the device when a certain limit is hit. You may want to check that with them as well. If the first person says no, or that they don't understand, then ask for a supervisor to find out.
If it turns out that they aren't (or at least they won't admit it) then try connecting as a hotspot rather than USB tether and see if that works. If it does, then you can access it the same way you would any WiFi connection. If it doesn't, then either the phone doesn't have the ability, ore we are back to the carrier/service provider causing a limitation.
Other than these, then you get into hardware problems.
If it is at all possible it would be done this way...
1. Go into "Settings" and in the "Wireless & networks" section tap "More".
2. Now tap "Tethering & portable hotspot".
3. For "Wireless Tethering" tap "Set up WiFi hotspot"
4. On this new screen you will see options for a "Network Name", "Security", "Password" and "Network Band". You can change them or keep them as is. It is really up to you.
5. Now you would just need to search for the hotspot by name using the second device.
NOTE: While a WiFi hotspot feature is built into Android, some service providers/carriers, especially in the United States, hide it. You would then need to see if you can locate an app that you can install to help with this.