After you file

Your W2 would have shown the $5K in Box 10.  With TurboTax, by default, the $5K you contributed to DepCare FSA is included in taxable income unless you match it with a $5K Child and Dependent Care Credit with details on the organization or person that provided the care.  This generates a Form 2441.  If this step for Child and Dependent Care Credit is the one you forgot, then you just need to open TT to amend your return and go to Deductions and Credits, and under You and Your Family, start/update the Child and Dependent Care Credit section.

 

Your question is confusing because you stated a deadline of March 31st.  This date sounds like a  DepCare FSA reimbursement deadline.