Internet is down but site are pinging

Hi

iam facing a different problem in my company on sunday from 6.00am to 3.30pm internet is working fine but after 3.30pm user’s complain me that internet is not comming but when i check ping command it is pinging but internet sites are not comming , so i started troublshooting i restarted router , i asked ISP on phone , but they are telling from your end firewall is blocking , i finally tired and configured temperorily proxy server ,next day my senior came and done something we get internet back but he did not revealed that information how he got the internet , can anybody explain me what might the exact problem

Regards
Meer Akbar Ali