The easiest and fastest way is for you to go back to your employer and ask for a duplicate. They should be able to accomodate you, but they are allowed to charge a small fee for providing duplicate copies of your W-2.
If the employer is not able or willing to help you, then you may contact the IRS for assistance. I am providing the IRS instruction for your circumstances, which are taken from the IRS website:
The IRS will help you obtain the your missing W-2 or 1099. Call 1–800–829–1040. Be prepared to provide your name, address (including zip code), phone number, Social Security Number, and dates of employment and the name, address (including zip code), and phone number of the employer/payer.
The IRS will send the employer/payer a Form 4598, Form W-2, 1098 or 1099 Not Received, Incorrect, or Lost. You will receive a copy, along with Form 4852 (PDF), Substitute for Form W–2 or Form 1099–R, and a Form 1040X (PDF), Amended U.S. Individual Income Tax Return. If you do not receive the Form W-2 or Form 1099–R in sufficient time to file timely, you may file using Form 4852.
If you later receive a Form W-2 or Form 1099–R, after your return is filed using substitute information, and the information differs from the information reported on your return, you must file an amended return.
Remember, the IRS will expect you to file a timely return, even if you do not have your W-2.