If you have lost your W-2, please contact your employer. Your employer can replace the lost form with a “reissued statement.” However, the employer may charge you a small fee for providing you with a new W-2.
The IRS states that "If you do not receive the missing information in time to file, you may use Form 4852, Substitute for Form W-2, Wage and Tax Statement". Taxpayers are requested to attach Form 4852 to the return, estimating income and withholding taxes as accurately as possible. Also, the IRS may delay the processing of your refund due while the information is verified.
Form 4852, Form 1040X, and instructions are available on the IRS Web site, IRS.gov or by calling 800-TAX-FORM (800-829-3676)