Do you mean that you didn't attach your W-2 to the tax return that you filed by mail, or do you mean that you did not include the W-2 income in the tax return that you filed?
If you filed your tax return by mail and did not attach your W-2, don't do anything. If the IRS wants a copy of your W-2 they will send you a letter asking for it. It could be many months before you hear from them.
If you filed your tax return and omitted the W-2 income you have to file an amended return to add the income that you omitted. But do not file an amended return until your original return has been processed and you have received the refund, if any. It's also possible that the IRS will notice the omitted income and send you a notice about it.
