You can prepare, print and mail a tax return at any time. However, you must have your W-2 or complete form 4852 as a substitute for the W-2 to mail with your tax return. The IRS will not accept your tax return without one or the other.
Here is an IRS article explaining how to get a lost W-2: https://www.irs.gov/newsroom/missing-your-w-2-heres-what-to-do
If you are able to get your W-2, you can still efile your return after entering the W-2 information.
