First you have to tell what changes you were made in numbering,IF you wants to change or modify your voucher numbering you can follow these steps:
Step 1:Go to the Administration menu,
Step 2:Click on Configuration,
Step 3:Click voucher series configuration,
Step 4:Click on sales and select series,
Step 5:Click voucher numbering,Here you can modify and set your voucher numbering automatically as you want.
Step 6:Click ok,then window will appear,If you want to set voucher numbering from the beginning,please select the Re numbering option as Yes,If you want the numbering to be automatic when you enter a voucher now,you can specify No in re numbering.