I have this problem of my internet in my pc unit (which I upgraded about 6 months ago) when it suddenly just losses connectivity while all the other computers in my house (including wifi) still have internet connectivity (My laptop can still connect via wifi, my mobile phone as well can connect). This seems to happen every 5 weeks or so, and the way i was able to fix it every time was to uninstall and install the ethernet driver. (I tried the classic modem power cycle along with the pc unit which doesn't work, which points to me that the problem is software-side as I was able to fix it via reinstalling the network ethernet adapter). It is a strange problem for me as I have never encountered this type of problem before. There might be some corrupt registry or file which may be causing it.
My area seldomly experiences power outages, but when it does, my PC has no UPS so it also shuts off. Not sure if it in anyway, corrupts any files in the network adapter. However, i have not experienced this type of problem from my previous PC builds.
Running Windows 7 64 bit, and the network adapter for my motherboard is MSI B85-G43
The loss of connectivity just happened today and I reinstalled the network driver, gaining back connectivity. This problem happens every 5-8 weeks.