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You need to contact the IRS and they will force the employer to send it to you, contact the IRS (1-800-829-1040). Be prepared to offer them the following information of the name and address of your employer, along with your name, address and social security number.
You are still required to file your return on time even if you do not receive a W-2. Attach Form 4852, Substitute for Form W-2, Wage and Tax Statement with your return. Any refund due you will be delayed until the information you provided is verified by the IRS and your employer.
Should you receive a W-2C (Corrected Copy of W-2) and you have already filed your return, you will need to file an amended return 1040X, Amended U.S. Individual Income Tax Return.
You will need to estimate the amount of salary and taxes paid by going through your last pay stub, and then file your return.