BYU scholarships and federal financial aid are treated differently when you serve a mission. BYU Scholarships you are awarded can be deferred while serving your mission. Federal financial aid is year specific, so any aid not used during an academic year due to leaving on a mission is lost. You will need to re-apply for federal aid by submitting the FAFSA upon returning.
Scholarship Deferment for Missions
For information on how how to defer your scholarship:
Loans and Deferments
If you took out a loan prior to your mission, you will go into repayment while on your mission. Please contact your lender directly to notify them of your plans and to ask about repayment options. Most lenders will happily work with you to make arrangements for a minimal or no monthly repayment option while you are away.
Finding Your Lender
Your federal loan records are available to view at studentaid.gov, including contact information for your lender (who you pay back ). Log in to studentaid.gov with your FSA ID.