Is there a federal law that allows management to remove nonexempt employees from federal service for taking overtime and all exempt employees to remain employed when they take overtime?

I was the timekeeper technician that input timesheets into the DISA system for federal employees, but there were 18 supervisors that gave me the timesheets for employees they supervised as well as their timesheet and they took overtime every pay period. When I took overtime and annotated on my timesheet my supervisor who was also one of the exempt employees who took overtime told upper management that he did not approve me to take overtime even though his signature was on my timesheet. He denied it and I was removed from federal service after 24 1/2 years of federal service. How do I fight this situation?