If you're match is postponed you can reply to your Full-Time text in order to change the game status on FA Full-Time website:
When you receive the text from FA Full-Time text service, simply reply "P-P" which will update the status of the match.
If you're not setup to receive SMS messages, please contact your league.
Please note: You can also postpone a match using FA Full-Time admin or The Matchday App guidance here: Postponing on Matchday App or Postponing on FA Full-Time