I just got a W-2 form from my old job that I did not know I still had tax withheld. I already filed my taxes. Is there a way I can file that W-2 form?

[HollyP]

To add another W-2 after you e-file you first have to have to wait until the IRS either rejects or accepts your return.  

1. Rejected. If your return is rejected, then you will be able to go into your account add the W-2 and re-submit your return.  
2. Accepted. If the IRS accepts your return, then you can you prepare an amended tax return and mail it in.  You cannot e-file an amended return.   

In the meantime, don't go in and start changing anything on your return in TurboTax, or you will make a mess for yourself.  Sit tight and wait until you see what the IRS does with the return you just e-filed.  

Form 1040X, Amendment will be available 2/11/16 and the following FAQ will contain instruction then to get started: https://ttlc.intuit.com/replies/3288565