forgot to mail W-2 form

Hi all,

 

I filed my taxes via turbotax with estimation of 500$ refund.
Few month later, I get a letter from IRS with due of 3500$!!.

Is the problem is the fact that I forgot to send the W-2 form? is that make sense?
How do I fix it?
Should I send an amend with W-2 form to fix it?

New in the US and very confuesed by all this,

Thank you for your help,
Shani