From Speedsolving.com Wiki

A DNS (short for Did Not Start) is the result of a solve that was not started at all. This is much rarer than the similar result of DNF, but it is considered to be just as bad - that is, a DNS is worse than any time and will count as the worst solve in an average. A DNS could be given if, for instance, a cuber is given a certain number of attempts for a BLD event but does not want to do all of them, or if they have to leave the competition before the current round is over. However, note that an average with two or more DNF or DNS results, a mean with one or more DNF or DNS results, or a best of round with only DNF and DNS results, is never considered to be a DNS, but rather a DNF. That is, the DNS result can only be given to single solves.