USB Tethering only SOMETIMES working
Need to narrow it down a bit, potentially three items assuming that your phone network operator is not at fault
Phone itself
Data cable
The PC
Can you if possible try your phone and cable on a different PC.
Do you have any data transfer issues with the phone on the PC ? such as backups etc,  worth trying another cable if possible.
Daft one but try the cable in a different USB port too.
Finally if you are able to try a different phone on your PC to test if tethering works.
It may not matter but others could need to know what OS your PC is running.