Hi Suresh,
1. First of all, determine which of the already existing document has the highest document number. For example, use the general table display for Table VBRK (Transaction SE16).
2. Then check the current number range status for the affected interval via the number range object maintenance (Transaction SNUM) for number range object RV_BELEG.
3. If necessary, increase the current number range status of the interval to the highest number already assigned, that is, to the highest number of the billing documents existing in the system.
Regards, Gordon