it just happened that one day when I am about to log into facebook page it says that facebook.com’s server DNS address could not be found. So i checked the computer setting if they blocked this page but it is not, what could be the problem of this.

Check your current DNS settings. From a command prompt with 'ipconfig /all' . What are your dns servers?

Once you have dns IPs, use nslookup to check.

nslookup
www.google.com
<you should get an answer here with IPs, if you don't, your DNS servers are wrong>
server 8.8.8.8
www.google.com
<you should get IPs here, if you don't, your outbound access is having issues going to 8.8.8.8>

Thank you so much pal :D