I have been running some tests to see if I can figure out exactly what causes loss of internet access.
In the Advanced Sharing settings, having Network Discovery on, makes a difference.
Whether you can ping your router or not depends on whether you have an valid IP address. If your IP address is not valid (169.xxx...) you should be able to set your IP address manually to be able to ping the router. If you do not set a DNS address, you will still not show internet access, but setting a DNS IP address will allow access. Some of these changes may take around a minute to take effect, so give it some time.
If you can't get an IP address automatically, then you are not getting a connection to the router or you DHCP server, or that server is turned off. Why you may not be able to reach the router, could be a firewall or anti-virus.
These comments are made assuming your Network adapter is installed and functioning properly and you have a strong wireless signal or a good wired connection. Also assumed is that no unusual circumstances are present, such as some required service on your system is disabled.