Update. I went to my router ip address and it turned out that DNS server was set to zero. So i set 208.67.222.222 in the Primary DNS Server box, and 208.67.220.220 in the Secondary DNS Server box. Now it works!
Someone gave me this advice. I don’t know if these DNS servers are good or bad, but it works now.
FurryNutz, I'll check all those things for my own educational purposes.
Thanks!