Here is a word from the Goodfellas at the IRS:
IRS Tax Tip 2013-10
If you have not received your W-2, follow these three steps:
If you receive the missing W-2 after filing your tax return and the information on the W-2 is different from what you reported using Form 4852, then you must correct your tax return. File Form 1040X, Amended U.S. Individual Income Tax Return to amend your tax return.
Forms and instructions are available at IRS.gov or by calling 800-TAX-FORM (800-829-3676).