Manually setting the IP Address and Other network related settings

The IP Address which i manually set few months earlier in the settings options of my browser was working fine till yesterday.But today when i restarted my system,i found that even if the LAN is working, i couldn't access inteet on my system.Then, i went to the settings option of my browser and changed the IP Address.Now,i can access inteet.

But i couldn't understand that why it happened that the IP address which was earlier set was working fine but not working today ??