This could be due to several reasons.

1. You have not be using the right nameservers normaly (ns1/

2. The domain of the nameserver may have expired.

3. The domain or the change, to the domain for the nameservers may have not have filtered through the whole internet yet. With most domain registers it takes 24 hours but depending with the domain you have be waiting for anything from 24 hours to 72 hours.