Hi,
1. Have you made any changes to your computer?
2. Do you receive any error message?
Method 1:
I would suggest you to follow the links and check.
iPhone: System requirements for Personal Hotspot
http://support.apple.com/kb/HT3574
iPhone: Troubleshooting Personal Hotspot
http://support.apple.com/kb/ts2756
I would also suggest you to contact apple support canter and check.
https://discussions.apple.com/community/iphone?categoryID=201