A President is elected if (s)he gets a majority of Electors appointed. The figure of 270 Electoral Votes needed to win is based on the assumption that every state appoints all of the Electors to which they are entitled.
If a state does not appoint Electors before the Electoral Votes are formally counted, then the number of Electoral Votes that would be needed for a President to be elected would decrease accordingly. If, for example, New York were not to appoint Electors, a candidate would need 255 Electoral Votes (a majority of 509) to win.
This is what happened in 1864, when the states under Confederate control did not appoint Electors, and two states that had recently returned to US control had their Electors rejected by Congress.
If a state is unable to certify election results, it would be up to the legislature of the state to decide whether to appoint Electors, and if so, on what basis they would be appointed.