If you forgot to include a W-2 with your return that you have already filed, it may not delay your refund, but unless you file an amended return to correct your mistake you will be contacted by the IRS for not reporting all of your income.
Adding the W-2 may increase your refund or it may require you to pay some money back. Without knowing your entire tax situation and the amounts that were not reported there is no way to know how your return will be affected.
When you create and file your amended return, it will take into account the refund that was shown on your originally filed return. It would be best for you to wait for the refund and then file the amended return so that the IRS will not be processing two returns for you at the same time. As long as the amended return is filed before the April deadline for 2017 returns, you will not incur any penalties for not including the W-2 on your return.
The instructions for amending your 2017 return will be available soon. The link below will update when they become available.
https://ttlc.intuit.com/replies/3288565
https://ttlc.intuit.com/replies/3318732
I did my taxes, but printed them out to mail in. Did not do efile, I just got another w2 last week that I didnt add what do I do
If you have not already mailed your return, you can add those w-2's that were just received and re-print your return to mail it in.