How is your system getting its DNS resolutions?
Do you have DNS servers configured in your router config or in your NIC config? If you don't and it is not pushed to your router from your ISP maybe try setting them yourself.
Use Google's DNS;
If you have custom DNS servers set either in your router config or your computer's NIC config, maybe those are failing. Try removing them.
Are you able to access the site from any other computer or network other than your own?
Are you accessing the website from your home network, company or school network, some other private network? Your website might be blocked!
Have you made any changes to your website's DNS record recently, which might still be propagating?