The answer varies depending on your league.
For recreational matches at all levels, if a referee has not been assigned to your match or an assigned referee does not show up, you still need to play the scheduled match. Ideally, find a parent who is a licensed "USSF Referee" and ask them to officiate. Otherwise, coaches need to determine before the start of the match who will referee the match. It is permissible to have two different people referee (one for each half of the match), as long as it was decided before the start of the match. The final match score stands and it needs to be submitted by the coaches to refereeadministrator@lwysa.org.
For any other league, follow the rules of that league. In general, because of the difficulty of rescheduling matches, most leagues follow the guidelines above. However, there are so many leagues with different approaches that it is best to just say "do what your league rules say".