What do you do if your employer doesnt give you your w2 on January 31?

Obviously its your employer and you should first inquire about whats up and give him a chance to make it right...very possible some other employee or system screwed up and a few days or so is needed. But otherwise, the steps officially are:

If you do not receive your Form W-2 or Form 1099-R by January 31st, or your information is incorrect, contact your employer/payer.

If you do not receive the missing or corrected form by February 14th from your employer/payer, you may call the IRS at 800-829-1040 for assistance. You must provide your name, address (including zip code), phone number, Social Security Number, dates of employment, your employer/payer's name, address (including zip code), and phone number. The IRS will contact the employer/payer for you and request the missing form. IRS will also send you a Form 4852 (PDF), Substitute for Form W-2 or Form 1099-R.

If you do not receive the missing form in sufficient time to file your tax return timely, you may use the Form 4852. If you receive the missing or corrected Form W-2 or Form 1099 after you file your return and a correction is needed, use Form 1040X (PDF), Amended U.S. Individual Income Tax Return. For additional information on filing an amended return, refer to Topic 308, Amended Returns.