You may get the original amount from your filed return. The IRS gets copies of all W-2's. When they match your return to the information you entered, they may send you a letter asking for the missing W-2.
You should wait until your original return is fully processed (refund received) before you amend. If the IRS does not notify you and you receive your original refund then you need to amend your return. Amended returns must be mailed.
