Reported, But Not Filed
You don't have to file an amended tax return for forgetting a W-2, as long as you included its information to figure out your tax burden. The IRS will send you a request for the missing document, and you'll provide it to the address specified. If you expect a refund, you may get it later than usual, as the agency may wait to receive the W-2 before issuing the check.
If you forgot about the W-2 entirely and didn't include it when figuring out your income, you'll have to file an amended return. Fill out a 1040X including that information, and mail the completed form – and a check for any additional tax owed – to the IRS as soon as possible. If you were due a refund from the first tax return you filed, wait to send the 1040X until you receive that refund. Regardless of how you filed your original return, a 1040X has to be filled out on paper. The IRS should already have a copy of the W-2 from your employer and matches it to the information you provide, so failing to include that information creates a discrepancy that increases your audit risk.