Your prep period can only be taken away in an emergency, such as a coverage for a teacher absence. You can lose multiple preps over the course of a semester due to an emergency need, but you will be paid for all lost preps at the current contractual rate of pay except one per term. So, you must do one emergency coverage per semester without being compensated, but you will be paid for all future coverages during the same semester.
To learn more about the policies regarding prep periods and coverages, visit the UFT website: https://www.uft.org/your-rights/know-your-rights/class-coverage.