You have to amend your return to include the W-2---and do not wait until you get a letter from the IRS. If your return has been fully processed and you received your refund you can amend it now.
https://ttlc.intuit.com/community/amending/help/how-to-amend-change-or-correct-a-return-you-already-...
Related Information:
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https://ttlc.intuit.com/questions/1908543-do-i-need-to-amend
https://ttlc.intuit.com/questions/2566895-what-does-it-mean-to-amend-a-return
If you do not have the other W-2 it is up to you to make every effort to get it from the employer.
You might need to phone, drop in, email and/or snail mail an old employer to make sure they know where to send your W-2.
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
