Most times it is an issue with the computer you are tethering to, you want to make sure your computer has all the proper ADB and USB drivers for your particular phone. If it is the phone that is not allowing tethering to be activated you may want to verify with your service provider that the tethering capability is allowed with your plan. (also 9 time out of 10 a prepaid plan will not alow tethering if by chance you have a prepaid plan) If you use a prepaid plan like I do, i recomment using tether from clockworkmod, but even in that case you still want to verify you have the proper USB drivers and you have USB debugging turned on. I hope this information can help you out.
IMHO guess the problem is with the carrier or plan has dropped or not paid up charge for tethering. The phone checks the network and account if tethering is enabled, and turns it off in 2-5 sec if network OR account says it should be off.