Win7 virtual wifi, connected but no internet access.

Hi. This is a very strange issue for me and confused me for a long long time. I have windows 7 pro installed, at the very beginning,I set up the virtual wifi and shared the internet to it. It worked well for mobile devices,very well. But after weeks or months the virtual wifi suddenly lost the internet access. I mean all the mobile devices could connect to this wifi hot spot but could...