You will want to contact your employer to get a copy of the missing W-2 so that you can have an accurate tax return. Although some employers are registered in our system to import the data, you would still need some additional information from the W-2 to import if that option is available. If your employer is uncooperative, then you will want to contact the IRS with your last check stub to let them know that you haven't received it yet and they will follow up with your employer or assist you with a substitute W-2. This will enable you to avoid having to file an amended tax return or having any IRS consequences such as penalties and interest.
To contact the IRS for a substitute W-2, follow the steps in the article below: