First, try contacting your employer and be sure they have your correct address.
Second, call the IRS at 800-829-1040 after February 23rd. Have handy your social security number and phone number, the dates you worked for the employer, and your last pay stub for the year. Also, give them your employer’s name, address and phone number. The IRS will send a letter to your employer for you.
And, lastly, file your tax return on time even if you don’t have your W-2 yet. Use Form 4852, Substitute for Form W-2 and estimate your wages and withholdings using your last pay stub.