My PC had the same problem several month ago. It turned out the DNS server setting of my PC was changed to to a DNS server in China by a unknown virus software. when you brower the youtube web site, the http request was redirectd to a website in China. I could not visit the youtube.com becuase the website is blocked in China.
please run the following commands on your to check the probelm
1. open a DOS window on your PC
2. type ipconfig -all to get the DNS server ip address.
3. run tracert command to find out if the DNS server is a server in China