If a woman does not menstruate for a few months, she does not have an obligation to separate until or unless she becomes a niddah again. (The exception to this rule is a woman who had a veset kavua for the Hebrew date, who would continue to separate on that Hebrew date until it had passed three times without incident.)
When she becomes a niddah again, she uses her first day of bleeding to determine her future days of anticipation.
If you are in this situation, it is advisable to check in with your health care provider to ascertain why menstruation has stopped. Please ask a follow-up question should you become a niddah again.