If you do not respond to an Office Action or submit your Statement of Use (SOU) (or request for an extension of time to file a SOU) before the filing deadline, then your application will be considered abandoned.
This means the application is no longer pending and will not mature into a registration.
Do not worry! Just because a mark has been abandoned doesn't mean you'll need to start all over again. As long as you file a Petition to Revive within 2 months from the mailing date of the Abandonment and pay the required government fee ($100 per class), you will retain rights in your application right where you left off and can file your Office Action response or make any necessary procedural amendments in order to get the mark ready for registration.