If your domainname is a gTLD with the extension (for example) .COM, .NET, .ORG of .INFO and the domain has suddenly stopped resolving, it's possible your domain has the "clientHold" status.
ICANN (Internet Corporation for Assigned Names and Numbers), is the instance regarding domain extensions such as .COM, .NET, .ORG & .INFO. ICANN is constantly improving the quality of her contacts. As a part of this, ever since 23 September 2014, all e-mail addresses of the owners of generic Top Level Domains (such as .COM, .NET, .ORG, .INFO and all new gTLD's) have to be validated.
If you don't validate your e-mail address within 2 weeks after a registration/update ICANN can place your domain in clientHold status, causing the domain to no longer resolve via DNS making it unreachable. Using a WHOIS you can check if your domain has this status.
How can I solve this?
To solve this, the best thing you can do is follow the steps in this FAQ: How do I verify my email address after registering a generic Top Level Domain (gTLD)? .
Take note! This only applies to gTLD's (generic top level domains) such as .COM, .ORG, .NET, .INFO, .PHOTO, .NINJA, etc. This does not apply to ccTLD's (country code top level domains) such as .NL, .BE, .DE, .(CO).UK, .NU, etc. If a certain ccTLD extensie does not resolve, this has to be caused by something else. The best way to solve this is to contact our support by sending a message through the control panel using the 'Contact' button.